Pull request
A pull request is a request to add the changes from one branch into the main version, so others can check them first.
In a simple example
You finish the new menu on a branch and open a pull request. A friend looks at it, says it is fine, and it is joined in.
Why it matters to you
It gives a second pair of eyes before a change goes live, which stops many mistakes.
Do not confuse with
A pull request does not change the live site by itself. The change happens when it is merged.