Ice blocks pieces, making it impossible to move them. You need to make a match or two (depending on the endurance level) with frozen pieces or detonate explosive power-ups to get rid of ice.
Ice can appear on elements like power-ups, shells, and tires in addition to normal pieces. In those cases, you can only remove it with an explosion.
Icebergs freeze neighboring pieces or power-ups with ice: one piece each move. You can’t get rid of or move icebergs.