...is the birth of the reader.
Death of the Author is a concept from mid-20th Century literary criticism; it holds that an author's intentions and biographical facts (the author's politics, religion, etc) should hold no special weight in determining an interpretation of their writing. This is usually understood as meaning that a writer's views about their own work are no more or less valid than the interpretations of any given reader. Intentions are one thing. What was actually accomplished might be something very different. The logic behind the concept is fairly simple: Books are meant to be read, not written, so the ways readers interpret them are as important and "real" as the author's intention. On the flip side, a lot of authors are unavailable or unwilling to comment on their intentions, and even when they are, they don't always make choices for reasons that make sense or are easily explainable to others (or sometimes even to themselves).
Likewise, as some critics note, it is elitist to assume that all artists are intellectuals or they have to be intellectuals, i.e., that works with deep meaning and ideas come only from people who are culturally and philosophically learned, rather than deriving from instinct, observation, creative inspiration, and artistic genius. Many consider this the Shakespeare authorship fallacy, i.e., that because Shakespeare was unlikely to possess the intellectual wherewithal to write his plays, the alderman's son from Stratford cannot have been the author of deep philosophical plays with dazzlingly complex characters. The academic consensus and textual studies overwhelmingly support William Shakespeare as the author and they note that whatever makes the plays deep comes entirely from command of language, stagecraft, and dramatic intuition, and while these skills can be intellectualized they are not innately intellectual, and while there's great depth, power, and meaning to a number of scenes in his plays the reasons for such meaning can vary between appealing to different kinds of audiences, subverting or parodying a convention that had already gotten stale way back then, or simple playfulness.
Although popular amongst postmodern critics, this has some concrete modernist thinking behind it as well, on the basis that the work is all that outlives the author (hence the concept's name) and we can only judge the work by the work itself. The author's later opinions about their work are themselves a form of criticism and analysis, and therefore are not necessarily consistent with what's written unless the author or publisher actively goes back and changes it—and it can still be argued that, since the original work still exists, the author has merely created a different version of it. One critic's understanding of the author's background and opinions is likely to be just as accurate as another's, especially if the author has an idiosyncratic or even anachronistic perspective on their own work. Modernists are more likely to appeal to the similar-yet-different concept of the Intentional Fallacy, which does not discount biographical information or other works by the same author. Playwright Alan Bennett claims he responded to students asking for assistance on analyzing his works as part of their A-Levels to "treat [him] like a dead author, who [is] thus unavailable for comment".
Needless to say, many writers as well as many other critics, don't like this or feel that this view holds true for all authors and all works. Margaret Atwood famously remarked that if the Death of the Author theory became prevalent, then "we [writers] are all in trouble". They also disagree with the implication that the Death of the Author/Birth of the Reader means that all interpretations are equally valid or that a reader's creative sensibility (whether it exists) is equal to that of a writer. Obviously some writers are more talented and capable than others, and certain works can only be written by some individuals. The notion also offends writers since it potentially leads to an overvaluing of the intellectual property of their works rather than the creative/legal rights of the author which has a contentious history in much legal and copyright disputes between creators and publishers. J. R. R. Tolkien acknowledged the influence of his experiences on his works (The Lord of the Rings), he denied that he had written allegory, insisting that his works simply had Applicability; this arguably makes him an early supporter of the Death of the Author, since pointless speculations about an author's allegorical intent are exactly what the concept seeks to avoid, favoring an analysis of the "applicability" of the text itself. On the other hand, Tolkien and his estate are quite protective of his works to ensure that it respects the overall basic intent of his work and restrictive over what filters in adaptations. In his essay "Creative Writing and Daydreaming" Sigmund Freud broached on the concept by noting that writers who work in popular genres tend to create works more reflective of the tensions and desires of the society as a whole than more artistic writers whose works mainly reflect their own sentiments and desires, which was an early attempt at qualifying intentionality in a work of art while also providing nuanced views on which kinds of works and authors display stronger intent than others.
Some people have noted that Roland Barthes, who actually wrote the trope naming essay, probably had to say "No, that's not what I meant at all!" at least once in his lifetime while discussing it. They have a point. In that essay, Barthes discussing a story by Honoré de Balzac in a very close reading simply noted how in the act of writing a complex work, Balzac's voice as author diffuses into multiple planes where one cannot know from reading closely if the narrative voice, character voice and plot voice truly expresses the author's perspective. By trying to do so, one cannot find it possible to extricate from these works some insight into Balzac's own thoughts, viewpoints and beliefs. For Barthes, the act of writing (and he meant writing only, with no hint as to how this trope applies to other media), allows the author to lose some of his conscious self and that for a work to be enjoyed, a reader has to project some of his own thoughts and views. Barthes' argument was based on close-reading i.e. the scattered random sentences in a story and other bits of detail, Rewatch Bonus and so on, not, as this trope is usually applied, on basic fundamental story beats and major plot points, in which the author's intent is far more conscious and clear. For Barthes, the idea that the author had clear and conscious intentions about every part of his work was dubious but not that the author had no intentions at all.
Barthes was also discussing a 19th Century author who while certainly popular did not write in genres with a vocal fanbase who had questions about everything and a medium to transmit those discussions and views to a wider medium. In modern times, on account of the growth of fandom and other conventions, some authors tend to be interviewed far more often than in the past, putting greater pressure on them to stay consistent. Some authors, such as Ray Bradbury and William Gibson can't be bothered to stay consistent when talking about the major themes or concepts in their books for more than a few years at a time. An author at a later moment, may come around to rejecting their own work, or express dissatisfaction with certain parts and not others. Hence, "the perfect is the enemy of the good" (i.e., "coulda, woulda, shoulda"). This is why some, but not all, auteur filmmakers oppose the notion of a Director's Cut on the grounds that the "real" film will always be the one people saw in cinemas in the year of release, not the ideal film in the director's head. This is a given in works where the authors don't hold copyright and can be replaced, especially Shared Universes; if a writer is fired and replaced by another, anything the old writer has stated in interviews can be (and often is) freely Jossed by the new writer.
There is an Older Than Feudalism example about some Jewish sages having an argument about their law...and ignoring God's interpretation in favor of their own. Because, you see, the Torah is not in Heaven. There is another in the Apology of Socrates: Socrates testifies that in his search for a wiser man than himself, he listened to the great poets. He thought their works very fine, but when they tried to explain them, he thought they were hopeless—and that the dumbest spectators around would do a better job. (He took this as proof that their poetic skills were a divine gift rather than an exercise of intellect.) It could be argued, however, that this hypothesis removes the only objective standard by which a text can be said to have a given meaning, or even any meaning at all. For since there are few times one could back up their interpretation of a poem with evidence, this hypothesis reduces all possible interpretations to mere subjective opinions (or at best, educated guesses). It might also be asked that, if it is meaningless for someone to say "That's not what I meant" when talking about any literature they might have written, then how can it be meaningful for any other situation where one might say that? How, for example, could a general criticize an underling for getting something absurd out of a set of instructions he or she may have given them? "Sir/ma'am, what makes you think you know what the orders meant just because you wrote them?"
Bottom line: A) when discussing a fictional work with others, don't expect "Author intended this to be X; therefore, it is X" to be the end of or your entire argument; it's universally expected that interpretations of fiction must at least be backed up with evidence from within the work itself and B) don't try to get out of analyzing a work by treating "ask the author what X means" as the only or even best way to find out what X means — you must search for an answer yourself, young seeker. Writing is the author's job; analyzing the work and drawing conclusions based on it is your job — if the author just gave away the answers every time, where would the fun be in that?
Related tropes include Shrug of God, The Walrus Was Paul (when the author encourages fans and critics to find their own interpretations), and Misaimed Fandom (which is what can happen when they do so). Often the driving force in Fanon Discontinuity where the fans dislike the author's interpretation to the point of ignoring it. This trope can be particularly useful and sometimes even encouraged in regard to tropes like Accidental Aesop, Broken Aesop, Unfortunate Implications, and others; see Warp That Aesop.
This trope does not mean "there is no such thing as canon for a work's events", which is a common misinterpretation of this theory used to justify Canon Defilement. We are completely aware of the irony in telling you how not to interpret it, but putting it in practice this way is just generally a bad idea. Compare this trope with Applicability and the Fiction Identity Postulate. A somewhat related trope is Word of Dante. Do not confuse this trope with Author Existence Failure, a literal death of the author.