This is exactly it for me: on your first reading, there are underlying concepts you kind of get, but don't fully understand until you grasp what's built on top

This is exactly it for me: on your first reading, there are underlying concepts you kind of get, but don't fully understand until you grasp what's built on top of them. Then, with that fuller grasp, you can go back and increase your understanding of the underlying material, which lets you understand the superstructure better — and the effect persists for several iterations if the topic is involved enough. Short version: don't be too concerned if you're not completely confident on the details the first time through, because it will make a lot more sense next time around.