Whenever you save your changes in the builder, either manually or via autosave, a snapshot of this version is saved as well. It's called a "Revision" and works the same way as revisions in WordPress. You can access all revisions by clicking the "Revision" (clock) icon on the left side of the panel.
Click on any revision to preview it. In order to use the selected revision and keep editing from there you need to click "APPLY". Otherwise the revision will be discarded.