Rollback is a special tool that allows certain users to revert vandalism and edits not in good faith, such as adding nonsense or gibberish, much faster. The Rollback function allows the most recent edits of a single editor to a page to be reverted using only one click.
For Staff with the right, the rollback link shows up in a user's contribution page, on the revision difference page, in the history of an article, and in the Recent Changes. Clicking the rollback link will undo all consecutive edits made by the most recent editor of a page. This is useful when reverting blatant vandalism such as page blanking, obscenities, and swear words. When the rollback is done, an edit summary will be automatically placed.
However, rollback shouldn't be used to revert an edit that looks like it was done in good faith or accidentally. The reason for this is because the person who made the edit won't know why it was being reverted if you use rollback. If you're not sure if the edit should be rollbacked then the rollback function should not be used, and instead opt to use the undo feature with an explanation given in the edit summary.