When a vibrating body swings forward, it pushes and compresses the air in front of it creating a region of high pressure. This region is called a compression. This compression starts to move away from the vibrating object. When the vibrating body swings backwards, it creates a region of low pressure called rarefaction. As the object swings / oscillates back and forth rapidly, a series of compressions and rarefactions is created in the air. These make the sound wave that propagates through air.