To understand something, you must actively engage with it
Learning is more than the transfer of raw information from media to the brain. In order to properly understand something, a combination of techniques should be employed in order to actively engage with the material. Regardless of the form of the information, this is critical for actually retaining and being able to employ this information later on.
Examples of this engagement include taking notes, discussing the ideas with others, writing essays in response, converting information to flash cards, etc. The material should be, in some way, transformed by way of this activity. This explains the design behind textbooks and university courses more generally; Textbooks are explicitly designed for learning. Similarly, university courses include tutorials and laboratories as part of the standard curriculum, which allow students to collaborate and work in tight feedback loops with tutors (subject-matter experts) to resolve questions and crystallise thought.