It is written in the 'Badges' page that the 'Cleanup' badge is awarded for the "first rollback". What does this mean? I could not understand. Can anyone help?
When somebody makes an edit to a post that is bad (wrong, defacing, spam, etc), somebody can come along and "rollback" the version to a previous one. Essentially throw out the edit and go back to a "good" version of the post.
This feature appears under the revision history of a post. You can view the revision history of a post by clicking on the circled link:
Once you click on that link, you will be shown the revision history for the post. The latest version and a difference of changes is shown. Previous versions include a "rollback" link which allows you to revert to that version. This is circled here:
It isn't shown here, but the mouse over text for the link says that this will make an older version the current one, undoing all changes newer and this is done to revert an "offensive" post.