The Meno is one of Plato's more famous Socratic dialogues (After The Republic and The Apology). It deals with the "Paradox of Inquiry", which is the question: "How do we know that we know something?"
The plot is fairly simple. Meno has been teaching people morality for a fee and Socrates (with his famous Socratic irony) asks him to teach him morality. Meno tries to do this but ultimately fails because Socrates isn't interested in learning morality, but, rather, is interested in proving that Meno cannot teach morality. Meno then compares him to a stingray that constantly stings others, leaving them unable to respond. Socrates likes that image, but, he believes that this stingray analogy works best if Socrates is also stinging himself. Socrates then explains one of philosophy's first theories of knowledge: the theory of recollection. By teaching a slave how to make a square with twice the area of another square using the Socratic method he proves (though, some scholarship debate whether or not this argument holds strong) that the slave always knew the theorem, that he just had to remember it. Socrates argues that everyone has an immortal soul that remembers all the knowledge we will ever need. So, we don't ever know anything knew, so, we don't ever have to know how we know something, we just have to recollect it, thus solving the "Paradox of Inquiry."
This theory of knowledge would later be refuted by Plato's more popular theory of the forms (which would later be refuted by Old-Man Plato). This dialogue, while short, is very important in the history of philosophy.