Different users have access to different functions of the site. While anyone can do most things on the site, including reading and editing, Rollbacks can access a few additional functions.
Anyone can revert vandalism and bad-faith edits, but it takes a couple of clicks in the page history to get it done. The "rollback" permission allows a user to undo bad edits with one click: by using the rollback link on diff pages, the user's contributions page, or the list of recent changes. The edit summary for a rollback edit is (Reverted edits by X (talk) to last version by Y).
Content moderators and admins have this permission by default. It can also be granted to other users by adding them to the rollback group. See Special:ListUsers/rollback for a member list.