Rarefaction is just the opposite, when molecules are given extra space and allowed to expand. Remember that sound is a type of kinetic energy. As the molecules are pressed together, they pass the kinetic energy to each other. Thus sound energy travels outward from the source. These molecules are part of a medium, or anything that carries sound.
Sound travels through air, water, or even a block of steel thus all are mediums for sound.
Without a medium there are no molecules to carry the sound waves. In places like space, where there is no atmosphere, there is no sound. Let's look at the example of a stereo speaker. To produce sound, a thin surfaced cone, called a diaphragm, vibrates back and forth, creating energy. When the diaphragm moves to the right, its energy pushes the air molecules on the right together, opening up space for the molecules on the left to move into.