That is a great question. This refers to an obstruction of the trachea, either above or below the vocal cords. It usually occurs in children because they have narrower airways. The most common type of extrathoracic obstruction we see in the emergency room I work in is croup, which causes swelling around the vocal cords and causes the child to have a barky cough that sounds like a seal, or a constant noise on inspiration called stridor. It can also be caused by an object like a hotdog getting stuck in the trachea, cancer around the traches, tonsillitis, or other process. According to our site, "Stridor may indicate an emergency and should always be evaluated immediately by a health care provider"
(To read more about extrathoracic airway obstruction you can check out this link.)