A thermal mass is a material that absorbs heat from a heat source, and then releases it slowly. This can be used to help regulate the heat in a space by increasing the amount of energy required to change its temperature. Concrete and bricks are often employed as thermal masses in a structure.
This is often used as part of a passive solar designed building.
Note: In order for a thermal mass to work it needs to interact with the air in the space, so cannot be covered.