Check out this handy dandy glossary for some helpful terms about the Khan Academy site
Progress levels: struggling, needs practice, practiced, level one, level two, mastered
Mastery challenge: Prove mastery on skills that you have already practiced in these challenges. Questions are a mix of concepts that you have done in the past. You might see a counter that says something like 15:56:32. This means you need to wait a little while (~16 hours) before another challenge is available. The time delay helps to ensure that concepts are retained over time. If you are using KA with students, encourage them to regularly do these challenges (e.g., at the start/end of every Khan Academy classtime).
Energy points: A measure of effort on Khan Academy. Points are awarded for different activities like doing an exercise, posting a comment, watching a video, or creating a CS program.
Avatars: Students enjoy customizing their profiles with these characters. Select your favorite, and earn more energy points to unlock more!