Rollback is a special tool that allows certain users to revert vandalism much faster. Normally, only admins can use the rollback function. Rollback rights are granted only to users that have contributed greatly to the Wiki, and are active enough to show they care about it. The rollback function allows the most recent edit to be reverted using only one click. The rollback link shows up in a user's contribution page and in the history between edits of a vandalized page. Rollback shouldn't be used to revert an edit that looks like it was done in good faith though. (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 get rollback, you will be titled Moderator.
If you want rollback rights, do not ask for them, just like you shouldn't ask to be a moderator on an internet forum. By contributing to pages and being active for long periods of time, you will get rollback quickly (mind the paradox).