Page History

...

rebase: the process by which you cut off the changes made in your local branch, and graft them onto the end of another branch. It also lets you reorganize or combine local commits in order to "clean up" your commit trail before you share it publicly via GitHub. For more information, see Pro-Git's chapter on Rebasing and GitHub's 'rebase' page.

...

If you would like to update your local checkout, for instance before sending a pull request for your local changes, git rebase is the tool you will use, e.g.
git rebase master

NEVER USE git rebase ON ANY BRANCH THAT YOU HAVE PUBLISHED TO GITHUB. If you do, it will likely cause issues for any other developers who are using your public codebase. For more information, read Pro Git's chapter on "Rebasing".

...