When a vibrating object moves forward, it pushes and compresses the air in front of it creating a region of high pressure. This region is called a compression (c) as shown in Fig. This compression starts to move away from the vibrating object, 

When the vibrating object moves backward, it creates a region of low pressure called rarefaction (R) as shown in Fig. As the object moves back and forth rapidly, a series of compressions and rarefactions is created in the air.