When you are in the middle of something, you learn that there are upstream changes that are possibly relevant to what you are doing. If your local changes conflict with the upstream changes, git pull refuses to overwrite your changes. In such a case, you can stash your changes away, perform a pull, and then unstash, like this:
$ git pull
...
file foobar not up to date, cannot merge.
$ git stash
$ git pull
$ git stash apply
