(covers information from several alternate timelines)
A security anklet was a device in use by Starfleet and Federation penal colonies to monitor the movement of the bearer of the device. The device is ring-shaped and is locked around the bearer's ankle. Wearing the device was said to be uncomfortable.
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")