Comment by dav_Oz

You are likely referring to ignorance: not knowing what you don't know thus resulting in a possible false confidence.

The article is about "feeling stupid" in hindsight. Because you cannot unsee it anymore. Which makes you wonder about other myriad obvious things you are missing.

"Feeling stupid" can also mean that you get the impression that everyone around you gets it but you don't.

The irony especially in maths - it seems - is that you can feel stupid because you don't get it and then quit but also keep feeling stupid if you finally got it because in hindsight everything falls so neatly into place that you can't imagine that you had so much trouble to get it in the first place!