Albedo is another name for reflectivity. The albedo of a surface determines how much sunlight will be reflected off a surface. A white, snowy surface will reflect more sunlight than a dark, rocky surface. Therefore a snowy surface has a higher albedo and does not change temperature very much when hit by sunlight. By contrast, a dark surface has a lower albedo and absorbs more sunlight, getting warmer.
Because darker surfaces absorb more sunlight and therefore get warmer, a positive feedback loop is created: The more melting there is due to warming, the warmer the surface gets.
