If signals can only flow in one direction at a time the communications is "half-duplex", like a single-lane road with traffic lights at each end. Walkie-talkies with a "press-to-talk" button provide half-duplex communications.
If signals can flow in both directions simultaneously the communications is "full-duplex", like a normal two-lane road. Telephones provide full-duplex communications.
The term "duplex" was first used in wireless, telegraph, and telephone communications. Nearly all communications circuits used by computers are two-way, so the term is seldom used.