This is a ice dam

When snow accumulates on a roof a cycle of melting and refreezing can occur. The snow melts on the surface of the roof as heat escapes from the attic through the roof. As the water runs down the roof it refreezes near the edge of the roof where heat escape is less and the surface of the roof is colder. This refreezing gradually builds up a ridge or heap of ice commonly referred to as an ice dam. The ice dam blocks the flow of melting snow, causing water to pool up behind the ice. When the ice dam reaches a certain size the water behind it will seep under the roofing, through the roof causing damage to the roof, attic, insulation, ceiling and walls inside the house. This situation can cause a lot of damage to a house and damage can occur quickly, especially as spring approaches and mild days cause water to pool up quickly.