What is a rollback?
A rollback reverts a post back to a previous version in the edit history. The rollback action itself then appears as the most recent item in the edit history.
A rollback also clears any spam or rude/abusive flags that have been set on a question. For example, if the post originally had two spam flags, was edited, received one more spam flag, then the edit was rolled back, the last spam flag will be cleared, leaving the first two.
Rolling back a post will also restore its content license (CC BY-SA 2.5, 3.0, or 4.0) to what it was at the time of the target revision.
How do I rollback a post?
You can find the rollback button in the revisions page:
If you don't have enough reputation to edit the post, you won't see the button. Also, there is no 5-minute grace period for rollbacks (i.e. any edits you make after the rollback will create a new revision, and not fold them into the rollback revision.)
How do rollbacks look like in the edit history?
Here's an example of what an edit history with a number of rollbacks looks like:
Can I provide a reason for rolling back?
The "Rollback" button will not provide you an opportunity to specify a reason for rolling back. However, you can provide a reason for rolling back by using the "Edit" button instead, and specifying your reason in the edit summary field. Do note that you must not change anything in the post for it to be counted as a rollback - if you make any change, it will be treated as an edit instead.
Rollbacks performed when overriding an approved suggested edit will contain an automatic summary that the edit approval was overridden.
If you're a moderator, and you accidentally use the Rollback button instead of Edit, you can change the summary of the rollback revision to add a reason for rolling back.
Do rollbacks affect suggested edits?
If there is a pending suggested edit on a post at the time of the rollback, the suggestion will be automatically invalidated (cancelled). Such suggested edits will not count toward automatic edit bans, much like automatic rejections due to edit conflicts, except the Community user will not be involved in the review. Such reviews show as "no longer reviewable" in the edit item itself, as "rejected edits" in the user activity summary, and as "invalidated" in the post timeline:
Rollbacks will not affect previous approved suggested edits: using the "Rollback" button in the revision history will only reverse the edit and take no other action, and is useful if a suggested edit is not useful but was made in good faith. Only those rollbacks which are performed using the suggested edit override button - and which are specifically noted as "Edit approval overridden" - will deduct the earned reputation and rewrite the edit as a rejected edit for the purposes of edit bans.