If you want to discard all unstaged changes, you can do so with this command:
git clean -f
This will revert all those files that are unstaged back to their old state.
Note that staging is when you use git add [file] to add a file to Git’s staging area in preparation for committing it.
If you want to discard files that are already staged, you can discard them with
git checkout -- [file] or all of them at once with
git checkout -- .