When you can store a bunch of information in your working memory as one unit. Eg. 2,0,1,9 is four numbers - but 2019 is chunked into one recent year. You can hold more information if you can chunk it up. Education should give you lots of chunkable abstractions. Chunks can be handled better by System 1.

Show deliberate practice of skill will help chunk that ability.

This is one of the ways learning happen.