Albedo is a measure of the reflectivity of a surface, and varies among different materials.

Light-colored surfaces have higher albedo, and tend to reflect heat away from earth.

Dark-colored surfaces have lower albedo, absorbing more heat and releasing the heat into the atmosphere.

Melting glaciers reduces high-albedo ice surfaces, exposing darker water and ground.

This becomes a positive feedback loop: the lower-albedo surfaces absorb more heat, and ice melt accelerates.