What is the role of rollbacks for badges on Stack Exchange?
Anyone told me it's used for badges; then, how do I get this?
Rolling back is the process of 'reverting' an edit (or several ones). Essentially, you're going back to a previous version of the post. You can only do this if you're the author of the posts, or have the edit privilege. You can do this via the revisions page, which can be visited by clicking the 'edited x min/hour(s) ago' link above the avatar of the last editor of a post.
Its name is derived from the rollback term in databases; a rollback cancels any changes made in the current database transaction.
Your first rollback on a site gives you the Cleanup badge, but you should never rollback just for the purpose of getting a badge. You should do it because the edit made the post worse, or the intentions of the author have been changed.