A snowball is a ball of snow. They are usually made by a person picking snow up in their hands and pressing it together in the ball the size of their hand. Snowballs are usually thrown at people or objects as a game called a snowball fight. Making snowballs is a common activity for children when it snows.
Larger snowballs are made by using a ball the size of a hand and rolling it on the ground. As more snow sticks to it, the ball grows larger. Larger snowballs are often used to make Snowmen. The term "Snowball effect" comes from this. It means as things go on, they get more serious in the same way that a snowball gets larger as it rolls.