The page history option allows you to roll back to a previously saved version of a page.
While in the editing view for the page you wish to roll back, click on the History tab in the top right-hand corner.
Down the left hand-side you will see a list of each version of the page, along with the date of the version, and the user who Published it. These are called Snapshots, click on a snapshot to preview it on the right.
Note: If you tick the Show unpublished versions checkbox, the list will also display snapshots from when users clicked on Save Draft. You can preview how the current snapshot looked on the site, by switching to Preview mode using the mode toggle in the bottom right-hand corner.
When you have located the snapshot that you'd like to roll back to, just click on the Revert to this Version button at the bottom.
The page will be rolled back to the selected version, note that the rolled back version will remain in draft until you click Save & Publish.