Subcutaneous locator beacons are beacons implanted under the skin of an individual, allowing them to be targeted by Asgard transporters, or tracked with handheld devices.
This also enables the wearer to be transported up to a ship usually via an Asgard beam and far quicker than usual. The devices first came into production in the aftermath of Dr. Daniel Jackson being captured on Tegalus which also saw him losing his Locator beacon.
As such, the new beacons were later introduced with SG-1, the main flagship team of Stargate Command being the first ever team associated with the Stargate Program to get the new implants before it was expanded presumably to all the other SGC teams. The beacons are manufactured by Devlin Medical Technologies. (SG1: "Off the Grid", "Memento Mori") (SGA: "Miller's Crossing")