This is a hypothetical device that can send signals to the past in theoretical physics. The idea is that, if a signal travels faster than light, it can be received, before it is sent. Thus, the signal has moved backward in time. However, tachyons, (the hypothetical particles that move faster than light) have not been shown to exist. This scenario remains hypothetical.
This image is of Paul Ehrenfest's 1911 depiction of a Minkowski diagram of tachyonic antitelephone.