A fork is when you take the source code of a project and then create a new one from it independently. To make it short, It's when you clone a project to make another one with some changes.

The term applies to cryptocurrency but it can be about a browser, an operating system etc.
For example, Qupzilla is a fork of Mozilla, Mozilla is a fork of Seamonkey. Kubuntu is a fork of Ubuntu, and so on. It's just a term in software engineering

As related to Bitcoin you can read this to understand better what it's about: https://blog.coinbase.com/what-is-a-bitcoin-fork-cba07fe73ef1