When something is released there will be people who like it. Those people are called fans. If the show/movie/ect. is popular enough then the fans will form a group. That's basically the definition of a fandom. That fandom will then form smaller groups depending on what people like about the show/movie/ect.
Now fans can be great people. If you like something that they don't, they are fine with that. If you don't know every piece of lore, they are fine with that. They make lovely artwork that makes me decide to change my point of view on a series.
But all fans aren't great people. Some can be downright toxic. If you don't agree with them then they go to war with you. One of my fanfictions on Archive of Our Own (A03) attracted the attention of a nasty Jon Snow/Daenerys Targaryen (Jonerys) shipper. They felt that I was doing battle with them as I don't like Jonerys in the adaptation of G.R.R.Martin's A Song of Ice and Fire books. While my opinion is if someone likes Jonerys, more power to them.
