"Wish fulfillment" means doing all of the things that someone wishes they could do. It's mostly used to talk about things like movies, games, books, TV shows, and so on.
For example, if there's a TV show about rich people wearing expensive clothes and driving around in expensive cars, that can be described as "wish fulfillment".
"Wish fulfillment" is actually negative. When you describe something as "wish fulfillment", it usually suggests showing someone getting a wish which is selfish, harmful, or silly.