Albedo

"The fraction of solar radiation reflected by a surface or object, often expressed as a percentage. Snow-covered surfaces have a high albedo, the surface albedo of soils ranges from high to low, and vegetation-covered surfaces and oceans have a low albedo. The Earth’s planetary albedo varies mainly through varying cloudiness, snow, ice, leaf area and land cover changes.""

When sunlight is reflected from something, that sunlight hasn't been absorbed so it doesn't contribute to warming that thing. So higher albedo has a cooling effect and lower albedo a warming effect.

Definition based on (revised from) IPCC AR4.