This is one of those ideas that seem completely obvious when you first hear of it, but once you’ve been made aware of it you keep noticing it again and again.
When learning something, you have to have a frame of reference in which to place the new piece of knowledge, or you can’t understand it. This is why trying to teach can sometimes be a very frustrating experience - you might think that something is completely obvious and can’t understand why your student cannot understand it, but that’s because your student doesn’t yet have the scaffolding required to hold up that understanding, scaffolding that you’re taking for granted. Whenever you are frustrated by someone’s lack of understanding, try to imagine what their scaffolding looks like and give them the next piece from the set of pieces that are missing.
This also sometimes sets the pace about how quickly you can learn something completely new and unfamiliar - there are lots of pieces of scaffolding missing and you need to take each one and internalize it before you can understand the next. Since it isn’t always obvious what the “next” piece should be, sometimes you have to read the whole textbook to get each piece. The problem isn’t memorizing lots of facts (though that helps) it’s slotting each piece into the framework.
If you’ve read the information about the next piece but haven’t yet internalized it, sleeping on it can help. When you dream your mind is playing a kind of tetris, sorting things out and slotting things into gaps so that it all fits together.
This theory also explains why young children want to have the same books read to them over and over again - they start off knowing nothing (not even how to learn) so they seek out familiar patterns. In the context of that repetition, a new piece of scaffolding will occasionally drop into place. When that happens, there is a satisfying “Ah ha!” feeling associated with it. We have somehow evolved a mechanism to recognize this event and derive pleasure from it in order to give us a drive for learning.