A dumbshow, also known as an empty is someone who has had their personality wiped without another one being imprinted in its place. They existed in a state similar to that of an Active's tabula rasa state, except that they were not able to function on their own, incapable of even speech. Dumbshows were also known to be susceptible to imprinting, as an active would be, and were created around the time that Butchers were.
It would appear modern technology now holds the threat of remotely wiping or imprinting masses of people, for instance by phone or via a "blanket signal". Topher Brink's unhinged rant suggests that a world power or an organization converted millions of people en masse into crazed killers (i.e. "butchers") and soldiers. Initially this was accomplished by means of a phone call, in the same way as Alpha remote-wiped Echo. Anyone who initially answered the phone became a butcher, and proceeded to mindlessly and psychotically kill anyone who didn’t answer. Later, China apparently acquired the tech required to imprint tens or even hundreds of millions of people all at once, with no direct connection needed. This technique, known as a "blanket signal", may have resembled the anti-wipe bomb that Topher detonates in order to affect a mass cure... It appears that this blanket signal converted people into dumbshows, as opposed to the initial phone call, which created butchers; a possible reason for this was so the butchers would have an easier time of killing the rest of the population, possibly as a prelude to a Chinese invasion, however the details of this are unknown... Even the general population of people who have escaped the technology, known as actuals, do not fully understand how it works.
Dumbshows are susceptible to remote imprints, either of people who are separated from their bodies (e.g. the case of Iris), or traditional chair-based imprints; this gives them the advantage of being able to be used to play back memories stored individually (as opposed to entire personalities or composite skill-sets), as in the case of Mr. Miller being used by the group of actuals to ascertain the cause of the apocalypse by playing back stored memories they found in Topher's lab in the abandoned L.A. Dollhouse.