A fandom is a group of people who share a common interest in something like a TV show, movie franchise, book series, video game, rock band, celebrity, sports team, etc. and do stuff to share in that interest with other people (I know that’s very broad but it can be stuff like collating/publishing news and speculation about their interest, writing fanfiction, attending conventions, cosplaying, making collages or compilation videos, etc.). Fandoms are a big part of “nerd culture”, and also often general teen culture; they’re influential in making there actually be fun content on the Internet and I also think they play a valuable role in pushing the envelope on our overbearing copyright laws.
At times I’ve dabbled in fandoms (mainly Doctor Who, to a lesser extent Harry Potter and Buffy, and if games count then certainly things like the Sims franchise and Neopets) but mostly I’ve been on the periphery, or an outright lurker. Fandoms can be full of ridiculous drama but they can also be a lot of fun.