Reading something and then reviewing it gives the feeling of understanding it but that is rarely the case. We normally can't recall what we read a few weeks after we finish a book.

Think about what happens when we are learning a new language. Getting good at reading and listening happens fast and gives us the impression that we know the language. Once it gets to talking and creating sentences on our own, we realize how far have to go to really know the language.

Highlighting a book and then reviewing the highlights is also not an effective way of learning. The good feeling from reviewing something comes from the familiarity of seeing it again. This can be confused with actual learning.

Learning is what happens when inputs are processed into outputs. Our brains have not really made the valuable connections until thoughts have been externalized.

This is why taking notes written with our own words is such a powerful way of learning. Implementing a spaced repetition writing system helps build a body of notes like this.