In school, I happened to notice exactly when some random topic went from "uh, I guess I kinda understand this" to having it actually "click". That clued me off to the difference between e.g. knowing a bunch of shitty formulae, and actually understanding the topic to the point where you can actually use it for problem solving. Also, that all the teaching and books I received revolved 100% around the former.
I had that moment when learning to code. I had like 98% or what I needed to know. Then one day I came across some random SO post, learned something that I really should have learned a while ago (the difference between static classes and instantiated classes, yes really) and then everything just fell into place and I realised I could actually write proper code now. It was a fun moment :)
For me it was variables. I know it's as basic as you can get here. But when I figured out that I could store data in a little box to get later I was like "oh, I can do anything with this"
Also, that all the teaching and books I received revolved 100% around the former.
This is true, because understanding comes from practice. The teacher and the books can never create that click of understanding. Only the student's side of the effort can do that.