A waldo is an electronic device Muppeteers use to remotely perform characters. Shaped like a Muppet's head, the waldo sends radio signals to a mechanism in the puppet that moves the puppet's mouth in sync with the waldo's mouth.
Waldoes were used prominently on Fraggle Rock (for the Doozers and the Gorgs). They were also used in most Muppet movies, The Jim Henson Hour, and various Creature Shop productions. The performer at the waldo usually provides the voice for the character.
Waldo C. Graphic, the computer generated Muppet, is performed entirely with a waldo; the device inspired his name