The description for this says it is: "First rollback"; however I have no idea what this means. Could someone explain?
This is a bit less prevalent now that edits are peer reviewed. However, if someone edited a post in such a way that was not productive, you could "rollback" the revision to the last good version.
Click on the "edited on ****" link on an edited post, and all previous revisions will show a set of links:
source edit rollback link. The rollback simply discards all revisions after the version that you are rolling back to.
The last time I remember using it was for undoing some excessive retagging of a question.