Network devices need to both send and receive information. In some cases, a device will send or receive at any given time, but not both at once. This is called half-duplex.
In other cases, a device can send and receive at the same time. This is called full-duplex.
In most cases, devices that use copper UTP cables are full-duplex. These cables have eight wires. Some wires are for sending, and others are for receiving.
Fibre optic cables can be either. Dual-core cables, that is fibre cables with two strands of fibre optic, are full-duplex. One core is for sending, and the other is for receiving.
Single-core fibre optic cables are half-duplex. They spend some of their time sending, and some of their time receiving.
WiFi is only ever half-duplex. It uses radio waves, and it can’t use a frequency for sending and receiving at the same time, as this would disrupt the signal.
Whenever possible, we prefer full-duplex, as it’s more efficient, and leads to better performance.