fork is a programming term, which means something like "copy the code and change some stuff and release a new version".
here is an example from github. this is the trending repository of the day:

if you click the fork button, you create a fork of this repository. then you can change what you want in its code and release your own version.
with bitcoin, your fork and change needs to be accepted by the majority of the people on this decentralized system to become bitcoin. SegWit was the fork that got this majority support.
bitcoin classic, bitcoin xt, bitcoin unlimited, bitcoin cash, ... are the forks that never got anywhere near majority support and that is why there is a lot of fuss about it. but you can always fork and even mine your own version of bitcoin as long as you have money to burn on mining, electricity,...