Your project folder is updated to the revision XXX. After that update, your working copy is not at the latest revision anymore and you can not commit any changes you make. If you want to commit, you have to update your working copy again to HEAD. Which means you can’t use this option to undo changes you’ve already committed but only to get a good look at an earlier time of your working copy. So: use this only to look, not to work.
All changes made up to the right-clicked revision are reverted (undo all from HEAD to rev). The changes are only done in your working copy, which you then can commit if you like.
Only the right-clicked revision is reverted. The revisions after the right-clicked one up to HEAD are not reverted. Use this if you only want to undo one single commit, not all commits since a specific revision. Again, since this is a revert: the changes are only done in your working copy, which you then can commit if you like.