There are lots of situations where a clean working copy is recommended or even required: when merging branches, when pulling from a remote, or simply when checking out a different branch.
The "git stash" command can help you to (temporarily but safely) store your uncommitted local changes - and leave you with a clean working copy.
―git stash: a clipboard for your changes
Your working copy is now clean: all uncommitted local changes have been saved on this kind of "clipboard" that Git's Stash represents. You're ready to start your new task (for example by pulling changes from remote or simply switching branches).
―git stash pop: continuing where you left off
$ git stash pop
The "pop" flag will reapply the last saved state and, at the same time, delete its representation on the Stash (in other words: it does the clean-up for you).