A class is a set of instructions that dictates how a specific instance of an object is to be created. A class is, in a sense, a recipe.
In the case of this book, a class includes details of authoring, editorial processes, and publication steps. These steps are, hopefully, invisible to anyone reading this book; they're part of the internal implementation of the class. Following these steps will produce an instance of the PowerShellBook object.
It's often necessary to look up the instructions for using a class in the .NET class library on MSDN, available at https://msdn.microsoft.com/en-us/library/mt472912(v=vs.110).aspx.
The starting point for creating an instance of an object is often what's known as a constructor.