(covers information from several alternate timelines)
A security anklet is a utility in use by Starfleet and penal colonies to monitor the movement of the bearer of the device, typically used with those who have committed crimes. The device is ring-shaped and is locked around the bearer's ankle.
On Earth, a person wearing a security anklet cannot leave the planet or tamper with the monitor without being prosecuted and charged. Tom Paris had to wear one while at the New Zealand Penal Settlement. (VOY: "Caretaker")
In an alternate timeline, Harry Kim was accused of conspiring with the Maquis after accessing classified files on the USS Voyager and meeting with Tom Paris in an attempt to determine how to return to his timeline. He was subsequently given a security anklet by order of Admiral Strickler. (VOY: "Non Sequitur")