Underlayment is, as the name suggests, a layer of material under flooring, whether laminate, carpet, wood, or another form. When you peel back the layers of your flooring, the underlayment will be sandwiched between the the flooring you see, and the subfloor. (Sometimes a moisture barrier is present as well.) The purpose of the underlayment is to reduce sound, offer cushion, and provide support to the flooring, among other benefits depending on the type of flooring. Underlayment can be made of any variety of materials from foam and cork to felt, depending on the flooring being installed over it.