Sometimes you might accidentally add a file you didn't want, or added it and don't want it staged for a commit.
You can undo this with:
git reset HEAD <file>
This will NOT remove the file. This simply unstages it and the git repo sees it as a "new" file" that can be staged. This is compared to:
git rm --cached <file>
This removes the file from the repo but doesn't actually rm the file itself. If --cached is removed, the file will be removed. Be careful and commit often.