Bronchiectasis isn't one single disease. Rather, it is a condition that results from injury to the bronchi and bronchial tubes. These are the networks of airways in the upper chest that deliver air into the lungs after it passes down the throat through the windpipe.
Bronchiectasis refers to damage to the bronchi that prevents them from delivering air properly. The walls of the bronchi become permanently widened (dilated), scarred, or inflamed. Some bronchi may even collapse. It is characterized by production of large amounts of sputum (e.g., mucus, phlegm). The condition may spread or only affect a small area. Usually bronchiectasis causes widening in medium-sized bronchi, but smaller bronchi can be scarred or destroyed as well.