We like to use concepts to help make sense of the world. Drawing arbitrary boundaries around one part of our experience so we can study it without having to think about the rest.
One of the fundamental divisions we use is Mind and Body. Psycho is that which pertains to the mind, Somatic is that which pertains to the body. Psycho-somatic is a term used to acknowledge an interrelationship between the two.
The problem is, there is no real boundary between body and mind. None at all. To study mind or body with the assumption that they are somehow separate places a large caveat on any knowledge we may gain through that study.
Boundaries may be necessary for developing detailed conceptual models of an aspect of the world. But we must remember that conceptual models are not the truth of the matter. Truth is revealed only when all boundaries fall away.
If we define mind as our thoughts, our inner landscape, and body as our physical form - skin, bones, tissues, organs, brain, nerves, etc, we can easily see why it feels natural to make a division. These two landscapes, on the surface, appear very different.
However, by developing attentiveness through practices like meditation & yoga we become aware of a continuous exchange taking place between these two realms of experience. Breath is seen to be one of the primary bridges. The state of our body conditions the quality of our breath, which in turn conditions the state of our mind. And the opposite is also true. This conditioning process is happening continuously, with every breath, in every moment.
We will also discover that the simple act of being attentive to the immediate experience of breath conditions all three - the breath, the body and the mind. The body will tend to become more open and relaxed. The breath will tend to become more subtle and refined. The mind will tend to become brighter and more easeful, the quality of thoughts will also change.
The more we develop the ability to rest in awareness, the more the boundaries between the three will fall away. Breath, body, skin, bones, tissues, organs, emotions, thoughts, ideas, are all seen to be just transient flavours of experience, intermingling in a continuous stream of energy, information, life-force.