If you don't understand an area of your lecture or a concept, ask the teacher!! If you feel funny doing this in class, then wait until after class. But don't just let it slide and make up excuses, like you can figure it out on your own or that it probably won't be on the test. There is nothing wrong with saying "I just don't get it". Once you do ask and the instructor explains it to you, you will feel so much better and less confused. It will even help you with other related topics and could help your test scores.