FCC alone isn’t the best way to learn JS: it’s expected that once you get some of the basics down that you look at other tutorial material. There’s also very rough spots in the curriculum that are IMHO just plain confusing, so to reiterate, outside tutorials are more likely to clear it up.
If you feel you don’t have any grasp on a concept, you might have a harder time with following challenges. On the other hand, seeing the concept in action might crystallize it for you, so don’t feel as if you have to have everything down solid before moving on. Go until you get stuck – you can always go back.