The phrase "main airway" refers to the largest and most important part of the respiratory system through which air flows. It is like the main road for air to travel in your body when you breathe. Full definition
The other type terminated in smaller endings farther away from the main airways, inside tiny air sacs called the alveolar space. (sciencedaily.com)
In dogs, most develop in the alveoli where oxygen is taken up into the body, but in people and in cats most originate in the main airways (bronchi). (brittonroadveterinaryclinic.com)
Canine Infectious Tracheobronchitis (kennel cough) is a highly contagious respiratory infection in dogs which results in swelling and inflammation of the main airways: the trachea and bronchi. (rachelebaker.com)