The correlative update system is a program within DHDs that tells when a Stargate has changed its position. Because of stellar drift, the update system will cause a Stargate to randomly dial the other Stargates to update its location to inform them of where they are.
Ba'al used the update system to send Avenger to the other Stargates, effectively shutting down the Stargate Network. Nerus later discovered a means of disabling the correlative update system, which Ba'al then intended to use to rebuild his empire. (SG1: "Avenger 2.0", "Off the Grid")