The wikipedia definition is this:
A zine (/ziĖn/ ZEEN; short for magazine or fanzine) is a small-circulation self-published work of original or appropriated texts and images, usually reproduced via photocopier. Zines are the product of either a single person or of a very small group, and are popularly photocopied into physical prints for circulation.
but the simplified is basically a small, usually magazine size/length comic or art book.
I'm not super knowledgeable about it myself but it's something I've been looking to get into since I've got the ability to sell online, with the goal of doing physical print further down the line.
I've seen fanzines pop of often on twitter where groups of artists collaborate or apply to join a collaboration either doing comics, short stories, illustrations and so on to put into zines and either sell digital or physical copies with profits going to certain places between the artists or charities.
I'm planning to do short ones, mainly solo for now, I'm just hoping to get a better idea of the process