An avalanche is when a large batch of snow on the side of a mountain slides quickly down the mountain. Because they are so sudden and the snow can move very quickly and with a lot of strength, an avalanche can sometimes be dangerous for whoever or whatever is in its path.
Avalanches usually happen shortly after a large snowfall (a foot or more) covers a steeply inclined mountain that already has a lot of snow on it. However, not every new snowfall triggers an avalanche factors like the temperature, angle and steepness of a slope, wind, vegetation (plants), and how the snow is packed all help determine whether or not an avalanche is likely to occur. Luckily, experts are sometimes able to predict the likelihood of an avalanche so that people in the area can avoid slopes or other places where they might be harmed. If you're ever on a ski slope or other place where avalanches are possible, remember to listen to the rules and advice of the grown-ups you're with!