Fix up one end of a slinky to a hook and hold it horizontally. Vibrate the other end to and fro along the length of the spring. If you look the spring carefully, you will see some parts of the spring are pushed closer , while some other parts are pulled apart.

Compressions are the regions of high density and rarefactions are regions of low density.These compressions and rarefactions result because sound is able to reflect off fixed ends and interfere with incident wave vibrates longitudinally; the longitudinal movement of air produces pressure fluctuations.