At the top of the Edit mode screen, on the left, is the "History and Autosave" tool.
This tool keeps track of all content that has ever been published on this page.
First, it tells you which page you are editing - just in case you forgot! There are so many possible interruptions that can happen in the middle of editing a page. You come back to the page and say "OK, where was I?" the page will tell you . . . 'Editing "page name": [ whatever ] version' . . . How handy is that!
When you first open the Editor, the tool will indicate that you are Editing the Published Version. . . well, probably.
As you work, the page will automatically Autosave every few seconds. Once the first autosave happens, the message will read Editing the Autosaved version.
When you publish the page, it is saved and the next time you edit the page the editor will again open with the message that you are Editing the Published version.
However, if you close the page - or move away from it by clicking some other navigation or Admin function - without publishing, the Autosave will be saved, but no changes will be made to the Published version of the page.
When you come back to edit the page, the tool will let you know that you are Editing an Autosaved version.
Note: because the autosave happens every few seconds, it is possible that a little typing or the addition of a block could happen between the automatic Autosave and when you leave the page. If this happens, those last few seconds of editing will be lost.
Every time you publish a page, a copy of the previously publish content will be saved.
This History is like a time machine that can take you backward and forward among all the versions of this page that have been published. If you have an Autosaved version available, it will let you move to published history and back to the Autosaved version.
Click on 'Autosaved' (or 'Published' if that is still displaying) You will see a list of options.
Clicking 'Published' will display the most recently Published version of the page.
Similarly you may choose a Published version from the history list.
You may re-publish a historical version. Because all versions are saved on Publish, you may also go back to the published version from before you reverted.
Remember that once you publish, the Autosaved version is no longer available.