Every time you open and close a Lesson as an Administrator, a new Revision is saved.
You can view all past versions of a Lesson by selecting 'Revisions' from the Lesson's card:
Here you’ll be able to see what time the Lesson was edited and who edited it. You can View (to see the content in that Revision), Restore (making an older Revision the 'current' version - the 'current' version is the one that will be Published when the 'Publish'), or Copy each Revision.