"The body is a unit."
Every part of the body is intimately related and so if any part of the body is not moving or functioning it directly affects the rest of the bodys ability to function.
In practice this means that in order to treat someone successfully you have to look further that just the muscle or joint that is actually hurting. If someone has a pain in their knee it is of great importance to understand how there foot or hip is moving. A limitation in the ankle joint will very directly affect the knee joint.