Activation index
From Halopedia, the Halo wiki
The Index, or Sacred Icon, as the Covenant call it, is an object found in every Halo's Library. It is the key needed to activate the Installation, causing a reaction that will purge the galaxy of sentient life. When inserted into The Core of the Halo, the Index activates Halo's main weapon. It is not known if an Index can be used to activate any Halo besides the one it is found on.
The Index is the key to Halo's main weapon, whatever that might be. It is needed to start the activation of Halo's final weapon sequence. Spartan-117 noted in his debriefing of the Halo incident that, if not for Cortana's intervention, he would have accidentally activated the weapon and destroyed all life in the galaxy. Current theories suggest Halo’s main weapon exists primarily in case the Flood manage to escape containment and spread their infection to other planets.
Summary
The purpose of the destruction of all sentient life is to starve The Flood, who cannot be stopped otherwise. The Index is roughly T-shaped, with a faint green glow emanating from a stripe on each side. It also seems to be able to be stored in a network, meaning that it is made out of an advanced material. Halo cannot be activated unless the Index is inserted into the control panel in Halo's control room. The "Sacred Icon" is first seen in Halo: Combat Evolved where, with the help of 343 Guilty Spark, SPARTAN-117 locates and attempts to activate the Halo, but he is stopped just before turning the key by Cortana. Cortana then begins to argue with Guilty Spark over the Index. After SPARTAN-117 breaks up their argument, Cortana explained to him the true function of Halo.
The Index appears again in Halo 2, although it is for Installation 05, when it is taken by Commander Miranda Keyes, but she and Sergeant Johnson lose it to Tartarus and his Brutes when they are ambushed. Tartarus then takes Keyes to Delta Halo's Control Room where he tries to force her to activate the ring, but fails and activates it himself (by forcing Miranda's hand) when the Arbiter and Johnson arrive. The Index is finally reclaimed by Miranda Keyes, who snatches it from the main platform after the Arbiter kills the Brute Chieftain.
The Index of Installation 04 was last known to be in the possession of Cortana and the Index of Installation 05 was last known to be in the possession of Commander Miranda Keyes.
It is called "The Key" by Gravemind in his lecture of the Chief and the Arbiter: "There is still time to stop the Key from turning", although with his manner of speech, it is not clear whether he was directly referring to the Index as "the Key", or just using a metaphor.
