An ice dam is a ridge of ice that forms at the edge of a roof, preventing melting snow from draining off. When snow on the roof melts due to warmer temperatures, the water can get trapped behind the ice dam, leading to potential leaks and water damage inside the building. Ice dams typically occur in areas with fluctuating temperatures, where daytime warmth melts snow, and nighttime cold refreezes the water. Proper insulation and ventilation in attics can help prevent the formation of ice dams by maintaining a consistent roof temperature, allowing snow to melt and drain away safely.