This book is intended as a correlative history of the modern soul and of a new power to judge.
This quotation represents Foucault's project in a nutshell. Discipline and Punish aims to chart the transformation from a situation where atrocious sentences were passed by a judge, and carried out in public, to one where the experts of the human sciences judge everyone in society according the norm established by their discipline. A new power to judge develops when psychiatrists, prison warders and other "technicians of discipline" begin to judge and analyze the criminal after the sentence has been passed. This new power is part of the development of a wider modern system of power and knowledge analyzed by Foucault.
The idea of a history of the soul is also an important one. One of the many models that Foucault uses is the shift of penality from a focus on the body to a focus on the soul. The modern system of discipline works on and attempts to reform the soul. Therefore an analysis of the development of modern discipline is also a history of the soul it created and attempts to control.