Git
Git is a tool that saves every version of your project, so you can go back if something breaks.
In a simple example
You change the menu page, save a version, and later see exactly what changed since yesterday.
Why it matters to you
It is your undo button for a whole project, and it lets many people work on the same files safely.
Do not confuse with
Git is the tool. GitHub is a website that stores Git projects online.