Wrist communicators were forms of communication worn on the user's wrist in the year 5000. The individual you were talking to appeared on the small screen.
Arden used one while reporting back to Leader Clent about the Ice Warrior's ship. Victoria Waterfield found Arden's on his dead body and used it to communicate to the Second Doctor and Clent. While being chased by Turoc through the glacier, Victoria dropped the communicator and when she tried to reach for it, Turoc grapped her wrist. Just then, an avalanche occurred, killing Turoc.
The Doctor used one while investigating the Warrior's ship. However, Varga realised he had some form of communicator and confiscated it. (TV: The Ice Warriors)