I see a Cleanup merit badge, which is described as "First rollback", but it has no information on how to earn it and no explanation as to what a rollback means? Is this deleting your earlier question or deleting a comment or answer? If so, we should state that in the right section.
A rollback is when you edit a post to revert it to an earlier version. If a post has previously been edited (such that it has at least two revisions), when you look at its edit history you'll see a
rollback link underneath the older revisions.
Revision 1 shows the
rollback link. Clicking on that1 would create a revision 3, which would be identical to revision 1.
For more information see the help center and some posts on Meta Stack Exchange:
1 Don't! Use this if you absolutely must experiment.