A good excerpt [slightly edited for clarity] from James Altucher's podcast discussion with Anders Ericsson about his book Peak: Secrets from the New Science of Expertise:
James Altucher: ...I always think the way to really retain the information from a book is if I go and tell someone "I just read this great book, here are the five things I've learned that are going to help improve myself."... I think that's the way I remember things later. By actually saying it out loud to somebody...I'm able to better remember the book as opposed to just trying to remember it without doing anything else.
Anders Ericsson: I think that's kind of the key that I see in all sorts of experts: their ability of actually mentally sort of think about things through reason and be able to now kind of work with it as opposed to this idea of just absorbing a lot of knowledge; and I guess the extreme case is just memorizing where you might be able to actually reproduce a book without understanding the main ideas. But I would argue that the real expert, they're extracting the main ideas--exactly like you were talking about--and making those ideas their own by relating it now to everything else that they know, and sometimes maybe even finding things that [they know] that seem to be in conflict with their generalization. And then I guess that leads to more thinking and discussion.
The excerpt above reminded me of the (apocryphal) story Charlie Munger likes to tell of Max Planck and his chauffeur. After winning the Nobel Prize, Planck toured around giving a speech. The chauffeur memorized the speech and asked if he could give it for him (pretending to be Planck) in Munich and Planck would pretend to be the chauffeur. Planck let him do it and after the speech someone asked a tough question. The real chauffeur said that he couldn’t believe someone in such an advanced city like Munich would ask such an elementary question and as such, he was going to ask his chauffeur (Planck) to reply.
As Charlie Munger mentioned in his 2007 USC Law School Commencement speech (Talk Ten in the Third Edition of Poor Charlie's Almanack):
In this world I think we have two kinds of knowledge: One is Planck knowledge, that of the people who really know. They’ve paid the dues, they have the aptitude. Then we’ve got chauffeur knowledge. They have learned to prattle the talk. They may have a big head of hair. They often have timbre in their voices. They make a big impression. But in the end what they've got is chauffeur knowledge. I think I’ve just described practically every politician in the United States.
And it also reminds me of Richard Feynman's discussion of the difference between really knowing something and just knowing the name of something (VIDEO HERE).