This question is showing an edit done almost an hour later. However when I clicked on the revision history, I got this:
What does this line mean? Rather, I want to know when it is shown? Also, what is the grace period?
The grace period (for edits) is a 5-minute window, in which all edits will be 'collapsed' into a single edit. You can hit the submit button as often as you want, it will still be just one edit. For details, see this tag wiki on Meta Stack Exchange.
When somebody edits an existing post, that will create a new revision. If they decide within those 5 minutes to rollback all their changes, there are effectively no changes, but the revision is not deleted. Instead, you'll see this message [Edit removed during grace period]. For the reasons why this is so, see the Meta Stack Exchange post @Void found: Undone edits disappear from revision history