Albedo is the fraction of solar energy (shortwave radiation) reflected from the Earth back into space. It is a measure of the reflectivity of the earth's surface.
Ice, especially with snow on top of it, has a high albedo: most sunlight hitting the surface bounces back towards space. (The word is derived from Latin albedo "whiteness", in turn from albus "white") Water is much more absorbent and less reflective. So, if there is a lot of water, more solar radiation is absorbed by the ocean than when ice dominates.
Albedo is not important at high latitudes in winter: there is hardly any incoming sunlight to worry about. It becomes important in spring and summer when the radiation entering through leads can greatly increase the melt rate of the sea ice.