One form this takes is that, even when you forget the details, you retain the general shape of the subject matter. You might not remember all the details about XYZ, but now at least you know XYZ exists: your internal map of the world is expanded and corrected, even if parts are a bit fuzzy.
A good way to think about it is the description of Mycroft Holmes by Sherlock Holmes;
"The conclusions of every department are passed to him, and he is the central exchange, the clearinghouse, which makes out the balance. All other men are specialists, but his specialism is omniscience. We will suppose that a minister needs information as to a point which involves the Navy, India, Canada and the bimetallic question; he could get his separate advices from various departments upon each, but only Mycroft can focus them all, and say offhand how each factor would affect the other. They began by using him as a short-cut, a convenience; now he has made himself an essential. In that great brain of his everything is pigeon-holed and can be handed out in an instant."