git pull origin master will pull changes from the
master branch and merge them to the local checked-out branch.
git pull origin/master will pull changes from the locally stored branch
origin/master and merge that to the local checked-out branch. The
origin/master branch is essentially a “cached copy” of what was last pulled from
origin, which is why it’s called a remote branch in git parlance. This might be somewhat confusing.
You can see what branches are available with
git branch and
git branch -r to see the “remote branches”.