A skintenna is an antenna surgically linked to a person that allows them to connect to the city grid. It can send pings and also play music that only the people on that skintenna frequency can hear.
Skintennas are generally worn by Special Circumstances agents, particularly the Cutters, that use them to communicate while working in the field. But they are also worn by the Swarm clique, as briefly mentioned in Pretties.
The skintenna is surgically placed in the spine of a human being, making it impossible for it to be removed without the death of the person wearing it. Skintennas can be turned off, however, as Tally did before she entered the Special Circumstances headquarters to confront Dr. Cable for the last time.
Skintennas can be turned off and on and given commands by making hand and finger gestures.