Some block types retain revisions where they remember some number of prior versions. These can be good to undo an accidental edit, an edit done by somebody who messed things up, or just when you change your mind and want it back the way it once was.
When they are available, revisions will be accessible with a "Browse Revisions" link. This will bring up a list of versions that you can browse. You can select any of them and "restore" it which will make a copy of it as the latest version.