He's saying that he made up categories and functors because what he really wanted to study was the idea of natural transformations, and the former notions are needed to define the latter. Or: categories and functors are nice, but natural transformations are the bomb.
People want to tell everything instead of telling the best 15 words. They want to learn everything instead of the best 15 words. In this thread, instead post the best 15-words from a book you've read recently (or anything else). It has to stand on its own. It's not a summary, the whole value needs to be contained in those words.
I'll start in the comments below.
(Voted by the Schelling study group as the best exercise of the meeting.)