1) Rarefaction is a region in a longitudinal wave where the wave particles are far apart. It is the region where the medium is spread out. They are regions of low air pressure. It is caused by a decrease in the density of air.
2) Compression is the opposite of rarefaction. It is a region in a longitudinal wave where the wave particles are close. They are regions of high air pressure. It is caused by an increase in the density of air.