The Nuva Symbols were six objects mysteriously connected to the elemental powers of the Toa Nuva. They served as the keys to the Nuva Cube which acted as the lock on the Bahrag's cage. The Toa Nuva lost their elemental powers when these objects were stolen by the Bohrok-Kal. The Toa theorized that the symbols needed to remain in the care of the Matoran for their elemental powers to remain available to them. The Toa Nuva regained the symbols following the Bohrok-Kal's defeat.
When the symbols of the Toa Nuva's might appeared, each of the Matoran villages pledged to honor and protect the symbol of its Toa Nuva for all time. Although Kopaka Nuva dismissed his symbol as an object with no power of its own, Matoro believed that the Matoran considered it to be a reminder that the Toa Nuva would always be there to protect them.
These strange symbols appeared in the villages of Mata Nui in the moment the Toa defeated the Bahrag. Little did anyone realize that these objects were not merely icons but were somehow linked to the powers of the Toa Nuva. When the Bohrok-Kal stole the symbols from the villages, the Toa Nuva lost their elemental powers. The Bohrok-Kal discovered that a floating cube served as the lock on the Bahrag's prison, and that the six Toa Nuva symbols were the key that could free the Bohrok queens.
Tahu Nuva's symbol was forged in the fires of the Mangai volcano. Gali Nuva's symbol was drawn from the waters of Gali's bay. Kopaka Nuva's symbol was as cold as the ice fields of Mount Ihu. Lewa Nuva's symbol was born of the living jungle. Pohatu Nuva's symbol was carved from the hardest stone. Onua Nuva's symbol was a treasure from the depths of Mata Nui.
The Nuva cube had shapes carved into each side, a perfect fit for the Toa Nuva's symbols. The Bohrok-Kal needed the icons in order to place them onto the cube and release the Bahrag, as the cube acted as a kind of lock.
The Toa Nuva only needed to retake possession of the Nuva symbols to regain their powers.