Similar to displacements of the particles, the pressure variations in the medium travel to the right.  Sound waves can be represented either by the displacement of the particles or by the pressure variations in the medium.

At this region, the sound wave separates the air particles apart.  The density is lower than the equilibrium density and, consequently,  the pressure is lower than the equilibrium pressure.  Such a low-pressure region is called a rarefaction.

Piston

Air particles

At this region, the sound wave brings the air particles closer.  The density and the pressure are more than their equilibrium values.  Such a compressed region is called a condensation.

The pressure variation is the pressure measured relative to the pressure of undisturbed air.  For example, Dp(x,t) = 0.1 Pa indicates that the pressure is 0.1 Pa above the equilibrium pressure and Dp(x,t) = 0 means that the pressure is equal to the equilibrium pressure.  Usually, the equilibrium pressure is equal to the atmospheric pressure.