Understanding a possibly complex idea in a way that allows you to reference it in a simple way.

A heuristic to identify if something is chunked is to see if you can explain it in simple terms. The Feynman Technique is a technique for Chunking.

It is possible that strong experts in their fields (mathematicians, etc) have gained that expertise by creating strong hierarchies of chunks that can then be manipulated and managed more easily.

This also relates to our ability to use our working memory. If we have sufficiently chunked an idea, it can take up less space in our working memory. Instead of all us having to remember all the details of the idea in working memory, a proper chunking means we have encapsulated those details in a more easily contained mental object. (I read about this somewhere but I’ve forgotten where now)

Has some structural similarities to Category Theory, where you abstract away the details of the individual objects in the category, and now only about the way they interact with each other.