A fan is someone who really likes something, like a sport or a celebrity or a TV show or a book series.
Being a fan means that they enjoy watching or thinking about that thing a lot. They might collect things like posters or t-shirts or toys that remind them of that thing. They might also like to talk to other fans about that thing and share their thoughts and opinions with them.
Fans can be very passionate about their favorite things and they might get very excited or emotional when watching or thinking about them. Some fans might even dress up as their favorite characters or attend events like conventions where they can meet other fans and celebrate their shared love for that thing.