@Mar's answer is right and perfectly solved this question, just add one comment.
if you want to rebase a branch based on remote master branch, git rebase origin/master is not enough, it will not get new commits directly from origin/master. You need to git fetch before git rebase origin/master.
or you can use another way to rebase a branch.
switch to master git checkout master
git pull origin master
switch back to your own branch git checkout {your branch}
git rebase origin/master
then, your branch is updated to newest commits.