The Autosave facility automatically saves your page, and places that autosaved version of the page into My Drafts. This will protect against work being lost, should you lose your internet connection or lose power, for example, while you are making changes to pages on your site.
Pages will be saved every 2 minutes, the first save creates the copy in My Drafts and each subsequent autosave updates the first copy with any changes.
If you see the message that a page is being edited by another person, when you go to Web Pages, it indicates that they have a version of this page in their My Drafts, that has not been published.
This allows editors to see if another editor has been making changes which have not yet been published.