It *is* difficult, but the authors of your textbook have tried their best to explain it and they probably have pictures to help explain.
We can try to explain here, using just text, but if the published author failed, how are we to do better -- with such a non-specific question?
What you can do is go to the library and look at how OTHER authors have explained it. You can see different examples there, too.
Meanwhile, a working database designer will rarely, if ever, have to think about the different levels. I realizes that a student needs to understand it to pass a test, but you can forget about it afterwards. The idea is to recognize when something is in 3rd-normal form. It becomes something that just seems obvious, once you've seen enough examples. OR, it might be something that you'll NEVER understand. Not everyone has what it takes.