There are two components to learning mathematics: learning skills and learning conceptual thinking.
Skills are learned through practice and drill. The fact that you understand the concept of parallel parking does not mean you can actually do it. You have to practice and practice and practice some more until you can do it. Once a skill is mastered, you never lose it; you may need some refresher practice after a long time of not using it, but that is all. Think bike riding. Skills are relatively easy to teach.
Conceptual thinking is a completely different thing.
The sequence of math classes from primary school through college form a continuum. At the lower levels, it is mostly about learning skills. At the advanced levels, it is mostly about conceptual thinking. In between, it is a mixture of both.