The bronchioles are tubes in the lungs which branch off from the larger bronchi that enter each lung, from the large and singular trachea which connects to the mouth.
As air is inhaled, it is drawn into the trachea and enters the main bronchus. The main bronchus then divides into two bronchi (one for each lung) which continue to subdivide before becoming bronchioles. As the bronchioles progressively fan out, literally like branches of a tree, they are each capped with an alveolus.
The trachea, bronchi, bronchioles, and alveoli all make up the lower respiratory tract. Each lung has around 30,000 bronchioles.