Heavy or massive objects like masonry, earth, and water can hold a lot of heat. Because of this capacity to act as a heat
(warming their surroundings) or a heat
(drawing heat from and cooling their surroundings), materials with thermal mass affect comfort both indoors and out. (Oceans and lakes, for example, moderate air temperature changes because their thermal mass acts as a buffer.)
Buildings in climates with large diurnal (day-night) temperature swings, like the high-elevation Southwest, offer a classic example of the time-lag effect of thermal mass. Adobe and other types of masonry walls absorb intense daytime heat, keeping temperatures comfortable inside. During the cold night, the walls pour out their accumulated heat, keeping the inside warm. By morning the walls, if they are designed correctly, can again absorb the daytime heat.