The bracelet itself can be worn around the wrist or neck (as a necklace), and has a small charm in the shape of a scythe on it. The charms are silver in color and can be easily concealed from other people as it can be worn as a charm or simply hiding it. The scythe is a symbol representing Kronos' scythe and is how Luke Castellan received information from his spies, as the scythe acts as a communicator.
The scythe charm appears during Percy and Beckendorf's attack on the Princess Andromeda. After they are captured by the forces of Kronos' army, Percy Jackson fought with the now possessed Luke and eventually gets injured when he is stabbed by Backbiter. As Percy tried to deal with his pain, Kronos let a silver bracelet with a scythe charm dangle from his hand and explains that he had known about the mission for weeks. Percy made the connection that the charm was a communication devise as Kronos commented that you can't count on friends. A moment later Beckendorf blew up the ship and Percy made it out safely.
Later while defending Olympus, Silena is injured by Drakon poison and began to die in front of Percy and her friend Clarisse La Rue. She admitted that she was Luke's spy and showed that she wore one of these charms on a bracelet around her wrist.
While no other scythe charms were mentioned in the series, some of the campers had known about one or multiple spies at camp since the events of The Sea of Monsters, where a camper told Luke the location of the Golden Fleece. They use this charm to send information.