From the back of the book:
The Star Systems on the outskirts of the Inner Sphere make up the area known as The Periphery. Extending far into interstellar space, it is fragmented into countless kingdoms and alliances. Small, mean, weak, and unorganized, they may yet tip the balance of power among the Successor States.
This ComStar report highlights the larger and more important states in The Periphery, detailing their histories, rulers, economies, backgrounds, and current military forces.