The Trachea is often better known by its functional name, the Windpipe. It is what connects the air-way of the Mouth-Pharynx-Larynx with the Lungs. It is lined with cells that produce mucus which traps particles to clean the air before it reaches the Lungs. The Trachea is flexible to allow bending and can collapse slightly to allow food to pass down the Esophagus behind.
In relation to vocal function the Trachea has the obvious responsibility of being the path for the air-pressure from the Lungs to the Larynx. Good posture helps to coordinate that relationship by ensuring the Trachea doesn’t bend causing interference. The Trachea is also an important part of the sensation of having an “open throat”. Becoming sensitive to the hollow feeling of the open Trachea helps to coordinate the proper positioning of the Larynx and the opening of the Pharyngeal space above. When well coordinated it gives the feeling of hollowness from the center of the head down into the chest, with the hollow feeling acting as one long resonator.
When this adjustment is combined with the phonating Larynx, the nature of sound vibration radiating in all directions (down as well as up) causes the sensation of chest or body resonance. So the Trachea is the physiological location of Chest Resonance.