Activation index
From Halopedia, the Halo wiki
Summary
The Index, refered to by the Covenant as the Sacred Icon, is the key to Halo's main weapon. It is needed to start the activation of Halo's final weapon sequence. If not for Cortana's intervention, the Master Chief would have accidentally activated the weapon and destroyed all life in the galaxy. These keys are housed in a certain place on the Halos known as the Library. It is speculated that the Index contains the codes for starting Halo. It may also contain information on which species need to be destroyed by Halo (hence the name Index)
