1. Compressions: These are regions where the particles of the medium are closely spaced together. In a sound wave, compressions are regions of high pressure.

2. Rarefactions: These are regions where the particles of the medium are spread out farther apart. In a sound wave, rarefactions are regions of low pressure.

Think of it like a spring: compressions are where the coils are close together, and rarefactions are where the coils are spread out.