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, such as the ocean, 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. Warmer temperatures cause ice and snow to melt into water, which is much darker. Because there is less ice and more water, less light overall is reflected and more is absorbed. This absorbed light makes the ocean warmer, which in turn makes the air warmer, which causes ice and snow to melt. With less ice and snow and more water, less sunlight is reflected and more is absorbed, causing the water to get warmer, and so on.