Fork
In plain terms
A personal copy of a repository you can modify freely.
Going deeper
Forking creates your own copy of someone else's repository so you can experiment or propose changes without affecting the original. It's the basis of open-source contribution.
Under the hood
Changes flow back via pull requests; central to the open-source contribution model.
Examples
Open-source contributions, experimenting, pull requests, GitHub forks.
Think of it like...
Like photocopying a recipe to tweak it without altering the original cookbook.