In the context of Bluetooth devices, “duplex” or “full duplex” means that two way communications are possible. Although a half-duplex system would provide communication in both directions, the signal would only be able to travel in one direction at a time (so if someone was speaking to you, you would need to wait until they finish speaking before you could reply.

A full-duplex system provides communication in both directions simultaneously, allowing signals to be sent and received at the same time.