A revision is a read-only (cannot be overwritten) version of your project.
To create a revision, click Save as --> A new Revision.
A count badge on top-right of 'Revisions' button helps you keep track of revisions of a project.
'Working version' is the project itself, working version can be overwritten by pressing Ctrl+S or clicking 'Save' button.
To list revisions of a project, click 'Revisions' button. This will open a dialog.
In 'Revisions' dialog, you can preview/rename/delete revisions.
Homepage lists revisions of a project, too.
Hover over a revision to preview it.