The buttons (in the upper left of this window) are called "navigator" buttons because they allow you to "navigate" through a lesson.

Go to the FIRST frame in the lesson
Go to the PREVIOUS frame in the lesson
Go to the NEXT frame in the lesson
Go to the LAST frame in the lesson

Each lesson is divided into "frames". Your current frame (and the total number in the lesson) is displayed below the navigator buttons.

The program "remembers" the last frame you visit for each lesson, so if you quit before finishing a lesson, you will automatically return to the right place when you start again.