How to Work with Versions in Google Slides
After you create a new presentation, every change or edit you make is recorded. The record of these changes is called the version history.
The Version history pane appears at the right showing all previous versions that have been auto-saved.
You see a preview of that version at the left.
Any changes that were made are highlighted in green.
To make it easier to know which version is which, you can give it a name.
The name is saved and now it's easy to tell which version this is.
Selecting a version in this pane doesn't actually bring this version back. You could still click the back arrow and return to the current version.
The older version is restored to the active version and you can continue making edits.