Summary
The Ark is a mysterious Forerunner device from which all the Halos can be activated. In the end of Halo 2, Delta Halo is armed and then unexpectedly shut down, causing it to send a superluminal signal to all other Halos in the galaxy, putting them on standby mode. 343 Guilty Spark informs Miranda Keyes and Sergeant Johnson that the Halos must now be shut down from "The Ark". The location of the Ark is implied to be somewhere on Earth, probably in New Mombasa. The Ark could also possibly be the Forerunner Ship.
It is expected that Halo 3 will reveal more details about the Ark.