There can be multiple authors within a course, and even multiple authors who want to contribute to a single lesson. Lt supports a single author in Edit mode at one time. If you attempt to edit a lesson that another author is working on, a "Lesson is locked" dialog will appear like the one shown below:
If you select Start Editing, it will lock the other author out of editing mode and they will no longer be able to make changes to the lesson. They will receive the following dialog showing them who has the current editing privileges:
This feature is very useful when an author forgets to close a lesson they were working on, as it means that no time is lost waiting for that author to return to their lesson and close it. Remember that other authors can lock you out of editing mode as well, so be considerate and use good judgment when taking over authoring privileges for a lesson.