Activation index
From Halopedia, the Halo wiki
The Index is an object found on every Halo. It is the figurative key needed to activate the "installation", causing a reaction that will purge the galaxy of sentient life. This removal of all sentient life is to starve the The Flood, who cannot be defeated otherwise. It is roughly T-shaped, with a faint blue-green glow emanating from a stripe on each side. 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) the Master Chief 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 the Master Chief breaks up their fight, Cortana explained to him the true function of Halo. (Previously Guilty Spark had implied that it would only destroy the Flood.) The Index of Installation 04 was last known to be in the possession of Cortana.
The The Covenant High Prophets regard the Index as a Sacred Icon.
