When the prongs of a tuning fork vibrate, they produce vibrations in the air; i.e., they set up back and forth movements in the air that correspond to the back and forth movements of the prongs of the tuning fork.
As the vibrating prongs strike the air, they push on it and thereby compress it. This compressing of a layer of air permits a layer of air just back of it to expand and therefore become less dense; thus a layer of compressed (denser) air is followed by a layer of rarer (less dense) air. Layers of compressed air and rare air follow one another. They are called sound waves. A sound wave consists of a compressed layer of air and the rare layer which is immediately after it.