Air bronchograms are something that many of us, even if we don't know ultrasound, have heard before. They refer to a consolidated lung that surrounds small airways.  This finding can easily be seen on ultrasound and is highly specific for pneumonia.  Learn the difference between a dynamic air bronchogram and a static air bronchogram with this video: 
Need some further reading? Look no further:

• A dynamic sign of alveolar consolidation in bedside ultrasonography: Air bronchogram.
• Lung ultrasound is an accurate diagnostic tool for the diagnosis of pneumonia in the emergency department. 
• The dynamic sonographic air bronchogram: a simple and immediate bedside diagnosis of alveolar consolidation in severe respiratory failure
• The dynamic air bronchogram. A lung ultrasound sign of alveolar consolidation ruling out atelectasis