Learning, understood as a relatively permanent change that comes from experience, runs through three stages: acquiring, retaining and retrieving. Each one can fail on its own, and that is why each one asks for something different.
Acquisition is the first contact: you take in the new information, encode it and start practising the basics. What counts here is attention. Retention is the move into long-term memory, what keeps something there even when you rarely use it. Retrieval is using what you learned when you need it: recalling a fact in a meeting, or performing a skill without having to think.
The trap is mixing up the stages. You walk out of a course feeling you understood everything, and you did: acquisition went well. A week later you try to explain it to a colleague and it won’t come out. Understanding held up; retrieval is what failed. Rereading your notes won’t train it, because recognising something and pulling it out of memory are different operations. What does train it is trying to recall without looking, with pauses between one attempt and the next.
The three stages are a useful simplification rather than three sealed boxes: what you do when you retrieve something also strengthens what you retain. Their practical value lies in diagnosis. When something won’t stick, first ask at which stage it breaks, and only then change the way you study.