Using the control that being a writer affords to create a story that addresses one or more outcomes that the author wishes would come about; that is wish fulfillment. There is nothing at all wrong with this, in and of itself. If Real Life fulfilled every psychological need we human beings have, we probably would not want to write fiction at all. Nor is there anything inherently unhealthy about using fantasy to compensate for a deficiency in Real Life; as long as one avoids becoming a Daydream Believer, Escapism and fantasy and Wish Fulfillment are healthy ways a human psyche can deal with dissatisfaction. Human beings have needs, and some of these needs are psychological in nature. This is an undeniable fact. When a person's Real Life does not provide all of these psychological needs, they can turn to fiction in order to provide for these needs. For instance, someone lives a very boring life going to an utterly mundane job and has no excitement at all. Said person's need for variation and stimulation is not being catered for. Thus, this person might feel very attracted towards stories where an otherwise-normal person they can easily relate to suddenly becomes an extraordinarily powerful being and is thrust into a wild and thrilling series of events. This, in turn, is why people sometimes defend fictional characters as if they were real, because they are not defending the character but idealized versions of themselves and/or embodiments of their values. An attack on the character is seen as an attack on traits they (the real person) personally possesses and/or admire. While it's a neutral term and an undeniable part of fiction in general, Wish Fulfillment is rarely mentioned in a positive light and with good reason. Usually, bringing up the term suggests that the author sacrificed quality in plotting, pacing, characterization, etc. in order to facilitate their own wishes. If the author takes it too far Mary Sues and Unfortunate Implications have a way of cropping up. However, it is entirely possible to have Wish Fulfillment stories that are still fluid and well-written; but these stories use only just enough to improve the story. Too much, and it stops improving the it and becomes indulgent instead. Conclusion being: don't worry too much about using these tropes as long as you mind well that balance. Contrast with Sour Grapes Tropes and True Art Is Angsty. Not to be confused with Wishing Tropes, which is about literal wishes being granted for characters in-story.
Tropes generally accepted to be interrelated (if not a direct cause-effect relation) to Wish Fulfillment: