Pregnant females use a den for shelter and to protect new-born cubs from the worst winter weather. Males will also take shelter from weather, but generally remain active in the winter.
The bears generally eat seals, which they hunt by waiting near the seal's breathing holes. The seal comes up for air or hauls itself out onto the ice and the bear makes a kill. This requires the bear to be able to blend into the white ice floe background so that it can get close enough to the hole in the ice. Hence the bear's white color. When the ice breaks up, the bears lose access to their primary food source. Remember that the ice stays around long after the worst winter weather is over. The question is how long it stays.
A basic FAQ is here.