The final tab is the Grades tab which gives you a spreadsheet-like view of assignments and grades, and lets you quickly input and edit scores and comments. Although you can work with grades from the Classwork tab, the Grades tab makes it easier to view and work with many assignments at the same time.
First, understand that the grades tab is not really a beginner tool - it becomes most useful once you are assigning and grading student work.
Optionally, to see an overview of the Grades Tab, watch the Introduction to the Grades Tab video.
If you haven't already, watch the video under The Assignment Cycle (the is the same tutorial from The Classwork Tab web page)