A sample grade is like a little piece of something that teachers use to see how well you understand a subject.
Let's pretend we're talking about math. Your teacher might give you some problems to solve in class, and then ask you to solve a few more on your own as a sample grade.
Then your teacher will look at your answers and see how well you did. If you got most of the answers right, your sample grade will be good! But if you got a lot of them wrong or didn't show your work, your sample grade might not be as good.
And these sample grades can help your teacher know what you need to work on and what you already understand.