Thermal mass is a term used to describe the ability of building materials to store heat (thermal storage capacity) whether the materials are inside or outside the building. Building materials that have thermal mass are able to absorb heat, store heat and release the stored heat at a later time.
Depending on which materials you use in the construction of a dwelling and type of climate you are living in, the thermal mass of the materials and/or combined materials that you use can make a big difference to the comfort level which affects your heating and cooling costs.
Concrete and bricks are high density materials so they are said to a have high thermal mass. But materials such as timber and cement sheeting only have a low density so they are said to have a low thermal mass.