Sound is a series of pressure pulses in air. Lets take a basic example: a bass-drum. the head of a bass-drum is a large circular membrane. When struck, the drum-head will flex and then return to its original position. At the moment when it flexes, it pushes the air which it is touching. This creates a zone of high-pressure immediately in front of the drum head. This high-pressure zone pushes against the adjacent air, and thus the zone of pressure begins to move away from the struck drum. This pressure wave propagates at at the speed of sound.
Human ears are devices which are sensitive to changes in air pressure. So when the pressure wave from the bass-drum reaches an ear drum, it will move the ear-drum in a manner which matches the shape of the pressure wave created when the drum was struck. The ear generates nerve impulses which are interpreted by the brain as sound.
The shape of the pressure wave reaching the ear carries information from which we decode the three major attributes of sound: Timbre, Frequency and Envelope.