Every Course contains a series of lessons. So user need to choose the desired course first by exploring course categories from explore course in left panel or from the top search.
User may find your desired lesson from top search section also.
In course overview tab, all lessons are arranged in a serial order. From there user should go through the start course or continue course button to start the first lesson or continue the last read lesson accordingly.
User may choose any lesson from course overview tab and go through it to read the lesson.