Merge
To merge is to join the changes from one branch into another.
In a simple example
After your friend approves the new menu, you merge it, and the main version now has the new menu.
Why it matters to you
Merging is the moment a change becomes part of the real project, so it deserves a last look.
Do not confuse with
Merge is not the same as deploy. Merge joins code. Deploy puts it online.