As it says, you should either commit or undo your changes. You can save them for letter with
git stash
when you want to reaply changes after stash, do: git stash pop
Or you can commit them with:
git commit -am 'Your commit message'
The last option, if you do not need your changes, you can simple remove them completely:
git reset --hard
Beware that you cannot restore changes after this command.
There are analogs of these commands in tortoisegit.