The authors draws our attention to a problem in the teaching of calculus: it's taught almost in reverse to how it was invented.
Examples from the book: among the concepts in calculus, integration was the first to be discovered and most intuitive. It has to do with the computation of areas and volumes and is easy to think about as computing by accumulation, i.e. adding together small pieces of a thing to find its size. Limits is the last thing to be understood by mathematicians.
In teaching calculus, however, it was done in reverse: limits comes up first, because formal maths teaching begins with definition.
The author's argument really struck a chord with me as I remember having almost exactly this conversation with my college math professor when I was an undergrad. What really bothered me was how we as students were presently a cleaned-up view of what math is. Hiding from us was the fact that, many of these ideas, and even notations, were confusing to the inventors and discoverers too. It took very smart mathematicians many years, often generations, to figure out what they were dealing with. I felt this knowledge alone would have helped me as a student.
More importantly, in the standard way of learning math, I wasn't introduced to the motivations that drove mathematicians to invent these ideas and tools. As the author points out in the book, often they had real, practical problems that needed solving.
This is a short book. The math examples given are easy to follow and intuitive.