Numbers to Leave Numbers, Form to Leave Form
These are concepts that I've first encountered by reading The Art of Learning by Josh Waitzkin.
These refer to the fact that, initially, when learning a new subject or skill, one has to learn the fundamentals very consciously.
For example, when learning to play chess, a beginner has to learn the names of the pieces, how each piece moves and what are the rules of capture, check and so on. Over time, the player stops dealing with those concepts consciously and starts living them.
To achieve mastery in any field, one must start by learning the fundamentals. Once the fundamentals are fully internalized, can they be left behind and more or less “forgotten”.
Once this happens, your attention can move on to focusing on higher-level concepts of increasing sophistication.