Jeff is a human who came to Equestria from nowhere turned into a pony, with no way to go back to earth, Jeff will have to stay "temporarily" in Equestria until he discovers a way to return to earth.
One of the things he learned about this world is that video games exist, but that they were very simple and boring compared to his world, and when he found out that his cutie mark had a connection to video games, he knew what he had to do.