The Sentinel is a planetary defense system on Latona.
The Sentinel is dependent on a human caretaker in order to function properly. The device removes invaders in flashes of light, presumably killing them. The machine is protected by an energy shield and to turn off the shield you have to find the right tone. But carapace code designs are constantly changing.
Approximately three hundred years ago, Marul's ancestors created the Sentinel as a planetary defense system; designed to protect the people while they pursued matters of the mind and spirit. The system successfully kept both the Goa'uld and other potential invaders from conquering the planet. (SG1: "The Sentinel")
By 2002, the Latonans had technologically regressed to the point where they had no clue how the Sentinel functioned. This posed a problem after a rogue NID unit killed the defense system's caretaker without whom it could not function. This left the planet subject to invasion by the Goa'uld Svarog.
The invasion was averted by SG-1 when Dr. Daniel Jackson realized the Sentinel's dependency on human life force. The device was reactivated when Grieves grasped the device itself, seemingly triggering an activation, and eliminated the Goa'uld invasion force. (SG1: "The Sentinel")