A fan patch is like a special helper that makes a computer game work better. When someone makes a computer game, sometimes there are mistakes or things that don't work the way they should. It's like if you were building a toy and accidentally left out a piece that makes it work well.
So, some people who really like playing the game want to make it work better. They create a fan patch, which is kind of like a digital band-aid for the game. They fix the mistakes or add new things to make the game more fun or easier to play.
Sometimes, game makers officially release patches themselves, but other times fans have to make their own because the game makers are too busy or don't have the same priorities as the fans. But it's important to remember that fan patches might not be approved or supported by the game makers, so you have to be careful where you get them from and make sure they won't cause any problems with your computer or the game itself.