Let me start by clarifying this term. It is a spiritual concept with profound significance. Whoever is in the center of your head is in charge of your life, making decisions for you such us, what career you choose, who are you going to marry, how you express your love to others, how you converse with God, if you can even talk to God at all, etc.
The center of the head is not only a metaphysical concept; it is also a very important anatomical and physiological region in our head. It is composed of two very important glands: the pituitary and the pineal gland. The pituitary gland is the master gland in your body and it is responsible of maintaining homeostasis as well as aiding developmental processes. The pineal gland is responsible for maintaining circadian rhythms in the body as well as regulating the secretion of hormones. Although scientific research shows otherwise, the pineal gland was considered in antiquity the "third eye" or the "seat of the soul."
Let me introduce a different concept here. Our bodies function due to a delicate balance between matter and energy. As you may remember from physics class, matter and energy are in balance in the Universe. They are mutually interchangeable. By this concept, if we start with the assumption that a change in energy in our body can affect its function significantly i.e., matter, what would happen if the influences of other people, i.e. energy, gets lodged into the center of our head, the place where two very important glands are located, the place where your insight originates? Scary thought, don't you think? I think I would very careful about who I let into my head. I'm not implying by any means that we shouldn't let anybody in, but we need to be aware of who we are letting in and that is what meditation can help us achieve.
What are the signs of not being in the center of your head or in charge of yourself? The range is very broad. On one end we find self-doubt, having to ask people's opinion about your most important and personal decisions. At the other end we see people who, as the expression says, are "worrying to death" or perhaps "dying of stress." Stress is nothing but other people's energy in our personal space, and widely known as the main contributor of life threatening diseases.
Now you understand why recognizing who is in the center of your head is so important. Chapter 1 in the book and meditation 1 on the website will explain more thoroughly how to be command and control of your body.