Rollback is a function that gives normal users the ability to use and have a rollback button without making them administrators.
Hypothetical Everything RollbacksEdit
While any user (logged-in or not) can revert a page to an earlier version, users with rollback and administrators have a faster, automated reversion tool to help them revert vandalism. When looking at a user's contributions, diff pages, recent changes, and page histories, a link that looks like: [rollback] – appears next to edits that are at the top of the edit history. Clicking on the link reverts to the last edit not authored by that user, with an automatic edit summary of (Reverted edits by X (Talk | block) to last version by Y) and marks it as a minor change. If, between loading the User Contributions page and pressing "rollback," someone else edits or rolls back the page, or if there was no previous editor, you will get an error message.
The rollback link on the diff page is somewhat misleading because reversion is not necessarily to the old version shown (the diff page may show the combined result of edits including some by other editors, or only part of the edits the rollback button would revert). To see the changes the rollback button would revert, view the corresponding diff page.
Rollback should be used with caution and restraint, in part because they leave no explanation for the revert in the edit summary. Reverting a good-faith edit may therefore send the message that "I think your edit was no better than vandalism and doesn't deserve even the courtesy of an explanation." It is a slap in the face to a good-faith editor. If you use the rollback feature for anything other than vandalism or for reverting yourself, it's polite to leave an explanation on the article talk page, or on the talk page of the user whose edit(s) you reverted.