It is not enough to just learn something by the text and even be able to pass any test. To really learn something, we have to familiarize it until it become intuitive.
What is intuitive?
Let's use an example, say "go to school". We may learn the definition of it, i.e. getting someone's physical body to a location of learning. And we may learn the interface of it, i.e. start at 7am, get out of the door, and expect around 7:15am be at school. However, try imagine yourself that you actually never did go to school and think about it -- you'll find that although you perfectly understand the definition and even the interface, you can't really feel it and therefore, remain non-ituitive.
What does it mean by "feel" it? Think about an object that you do have feel, e.g. your coffee mug. Think about it, you know your coffee mug not by definition nor the interface, rather you possess a rich store of sensational experience that when you close your eyes you can refer to to derive information. So to be intuitive, we don't necessarily need definitions or interfaces, but we need sensational experiences.
Back to "go to school", now do recall how you went to school rather than the definition of it. Immediately, there probablly will be a feeling of how hard it is, how long it is, how pleasant or how boring it is, etc. As we think about it, it appears we can ask infinite amount of questions and retrieve infinitive amount of information out of our experiences. And we know that without actually going over these questions -- we have a "feel" for it. When you have a feel for something, we no longer spend any trouble going over any details, rather, we develop the confidence that we are able to retrieve the details anytime we want to; and as a result, we glide over the concept, which become intuitive.
So for "go to school" to become intuitive, it is not sufficient to acquire the definition and learn the interface, it is necessary to experience it -- not necessary to experience all forms of it, but at least a few of them with actual feel.
I learned quantum mechanics in college -- actually a few times over different courses -- but the concept of states and Hamiltonian remain non-intuitive to me. Only after I actually implemented some computation in code myself, now I find the ideas intuitive. Not only I can defined them if I want to, I physically feel the details as matrixes, vectors, and as a physical code that takes certain amount of time to compile and patience to run -- I can feel them so now I don't have to recall them.