Icebergs are large floating mountains of packed ice at sea. They move around slowly in icey, frozen water. At least 70 or even 90 procent of the iceberg isn't visible, the largest part is under water. Usually they are 30 metres high or more. But they can be smaller.
They cannot be underestimated as they could be harder than rock, and could cause serious damage to ships. They are often a problem in the North Atlantic, where the most important shipping routes lie between Europe and America.
It's what sank the most famous ship in the world, RMS Titanic. Since then, they're taken more serious.