Reading for Information vs Reading for Understanding
Adler and Van Doren separate two very different outcomes that ordinary language collapses into one word: information and understanding. Reading for information gives the reader new facts or reports inside a framework he already has. Reading for understanding changes the framework itself.
This is why hard books feel different. If a book is genuinely above your current level, then merely "getting the facts" is not the main job. You are trying to rise from less understanding to more understanding. The book is not just handing over content. It is forcing reorganization.
The distinction explains why effort matters. Informational reading can often be quick, smooth, and low-friction. Reading for understanding is slower because the reader has to close a gap between what he can already follow and what the author is actually doing. In that sense it connects tightly to prereq mastery and focused vs diffuse thinking: sometimes the mind has to wrestle with something before it can integrate it.
The concept also helps explain why so much modern reading produces the feeling of learning without much real transformation. A person can consume endless informational material and still rarely undergo the kind of restructuring that difficult books demand.