Explain the meaning of terms compression and rarefaction in relation to a longitudinal wave.

Compression - When a vibrating object advances, it pushes and compresses the air that is in front of it, thus creating a region of high pressure, the region is called compression C as observed in the figure. The compression begins to drift away from the object that vibrates. When the vibrating object moves backwards, it creates an area of low pressure known as rarefaction as observed in the figure.
Rarefaction is the region of low density, wherein the particles of the medium move away from each other, compression on the other hand, are the regions of high density where the particles of the medium are very close to each other.