https://en.wikipedia.org/wiki/Duplex...ommunications)

A duplex communication system is a point-to-point system composed of two connected parties or devices that can communicate with one another in both directions, simultaneously. An example of a duplex device is a telephone. The people at both ends of a telephone call can speak at the same time, the earphone can reproduce the speech of the other person as the microphone transmits the speech of the local person, because there is a two-way communication channel between them.
A half-duplex (HDX) system provides communication in both directions, but only one direction at a time (not simultaneously). Typically, once a party begins receiving a signal, it must wait for the transmitter to stop transmitting, before replying (antennas are of trans-receiver type in these devices, so as to transmit and receive the signal as well).An example of a half-duplex system is a two-party system such as a walkie-talkie, wherein one must use "Over" or another previously designated command to indicate the end of transmission, and ensure that only one party transmits at a time, because both parties transmit and receive on the same frequency.
A good analogy for a half-duplex system would be a one-lane road with traffic controllers at each end. Traffic can flow in both directions, but only one direction at a time, regulated by the traffic controllers.