The curse that Toji has with him is called the Inventory Curse. It is a very special, worm-like cursed spirit with a human head that can be used to store basically anything, from weapons to other curses, as in the series. And while it is usually big enough to wrap itself around Toji’s torso, it can become so small that Toji can hide it inside his mouth and body, thereby hiding its presence from the other jujutsu sorcerers. Aside from storing stuff, it doesn’t seem to have any other powers, but it can talk.