Branch
A branch is a separate copy of your project where you can try changes without touching the main version.
In a simple example
You make a branch called new-menu, try a new design there, and keep the live site safe until you like the result.
Why it matters to you
You can experiment freely, and a failed try never breaks what customers see.
Do not confuse with
A branch is not a backup. It is a side road that you later join back, or throw away.