A cool roof is made of a material or has a coating that can lower the roof surface temperature, decreasing the amount of heat transferred into a residential or commercial building. In general, traditional roofs absorb sunlight during the day, heating the building and the surrounding air. Roofs that are lighter in color or are reflective stay cooler than roofs that absorb sunlight.
https://www.energystar.gov/products/cool_roofs_emissivity#:~:text=A%20cool%20roof%20is%20made,building%20and%20the%20surrounding%20air.