Ice blocks pieces, making it impossible to move them. You need to make a match, or two (depending on the number of layers), with frozen pieces or explode power-ups to get rid of Ice.
Ice can also appear on elements like power-ups, Shells, and Tires. In those cases, you can only remove them using by setting off an explosion.
Icebergs freeze adjacent pieces or power-ups with Ice: one piece with each move. You can't get rid of or move Icebergs.