Duplex communication is the concept that information can flow in both directions across the cable. On the flip side, a process called simplex communication is unidirectional.
Think about a baby monitor, where the transmission of data only goes in one direction making it a simplex communication. A phone call on the other hand is duplex since both parties can listen and speak.
The way networking cables ensure that duplex communication is possible is by reserving one or two pairs for communicating in one direction. They then use the other one or two pairs for communicating in the other direction. So, devices on either side of a networking link can both communicate with each other at the exact same time. This is known as full-duplex. If there’s something wrong with the connection, you might see a network link degrade and report itself as operating as half-duplex. Half-duplex means that, while communication is possible in each direction, only one device can be communicating at a time.