In the description for the cleanup badge, it says to earn this badge, you need to have your first "rollback", yet when I search rollback on the internet, I get "a reduction or decrease". I do not understand what this means. May someone please explain this to me?
See this meta question for the specifics. Essentially, you and other users are allowed to edit your posts (questions or answers). Sometimes you change your mind about your own edit, or you disapprove of another's edit. In these cases you can "rollback" the edit, returning the post to its state before the edit was made.
As an example commonly seen on Puzzling.SE, suppose User A creates a puzzle, and User B edits User A's puzzle to fix some typos. But it turns out, that User A intended for there to be typos in order to hide a clue relevant to the puzzle. So User A may rollback User B's edit.
It seems you have mistaken the "Critic" badge for the "Cleanup" badge. To get the critic badge, you must place a downvote. (Please don't do this to a random post just to get the badge.) To get the cleanup badge, you must rollback an edit.