The lesson navigation menu allows you to switch between lessons, quizzes, assignments, etc., without having to use the back arrow to go back to the Content tab of the course.
The expanded lesson navigation is shown below.
The arrow at the top right pointing left allows the lesson navigation to be collapsed.
The next button allows the lesson navigation to be minimized.
The folder icon above the list of lessons will expand/collapse all modules.
The Preview Lesson button will place the lesson in preview mode.
The Exit Lesson Edit will go back to the Content tab of the course.
Minimized Lesson Navigation
Collapsed Lesson Navigation
Lesson Navigation with Modules collapsed