The body is not just a shell or a device carrying out instructions from the brain. As the author explains in the book, whatever we do from the neck down has a great impact on what happens from the neck up. Because of this, our bodies have great power in shaping the development of our minds.
Beilock mentions too many examples of the importance of how the body affects the mind (and, by the way, how your interaction with different environments, such as being in nature, affect your mind) to discuss them all, here (buy the book, I would say). As Beilock writes, the knowledge presented in this book can be used to (1) feel beter, (2) think better, (3) develop your mind, and (4) to understand other people.
I think this is a great book.
Also read: Physical activity and brain health