Tofu (or bean curd) comes in many different forms, frozen tofu is exactly what it sounds like. It is regular firm tofu, that is steamed and then frozen.
Since tofu is stored in water, ice crystals form, making porous, sponge-like holes in the block. This allows for a chewier texture and more flavorful.
The sponge-like texture of frozen tofu allows it to absorb all the flavor from the soup, makes it a common ingredient for many hot pot and soup dishes.