a ballast is any heavy material used to hold down another material.

Most commonly, ballasts are used on roofs. A ballasted roof is one where the roof membrane is not mechanically attached, or adhered. It is simply laid on top of the insulation or substrate, and the ballast (most commonly gravel or smooth rocks) are laid on top. The ballast allows for drainage, protects the membrane, and reduces solar gain, but adds a significant load which must be supported by the superstructure below.