A low-pressure region is being created between the prongs of the tuning fork.
Work Step by Step
As described on page 377, the situation may be pictured as a Ping-Pong paddle (the prong) moving in a roomful of Ping-Pong balls (the air molecules). When the paddle moves one way, gathering Ping-Pong balls closer together in front of it, balls behind it move in to fill the space just created, leaving more space between them. As the prong vibrates, it creates an alternating series of compressions (high-pressure regions) and rarefactions (low-pressure regions).