Ice damming happens when the heat in your home escapes into the attic and warms the roof causing the snow on top of it to melt.
Because hot air rises, only the top of the roof gets warm; and when that melted snow runs down the roof it turns to ice when it reaches the cold overhangs. After a while that ice becomes a dam, pushing water back up the roof and under your shingles. Before long, you have a leak.
To prevent this problem, you should seal any area where warm air can leak into the attic, and be sure you have a continuous blanket of attic insulation that’s adequate for your climate. You should also be sure that soffit vents aren’t blocked with insulation because they let fresh air into the attic, which helps keep the entire roof a consistent temperature, minimizing ice dam effect.