Basically it says that each learner has to construct knowledge/understanding in his/her brain, and that you can't really learn something unless it "fits into" a gap in your mental model, so it can connect to other things in your neural net. (Well, that's the main way, assimilation. There's a harder way to learn, accommodation, which is what happens when a new input /contradicts/ something you already know, and instead of forgetting the new thing, which is what usually happens, you rearrange your old knowledge to form an understanding that's compatible with the new thing.)