Uart Full Duplex vs Half Duplex
Simplex, half duplex and full duplex shown below: -



Half duplex may make use of separate transmit_left and transmit_right wires or use some kind of hardware to permit non-simultaneous left or right transmissions on one shared wire (plus ground).

Full duplex can look like half duplex in terms of wires but, the hardware at both ends can handle simultaneous transmission and reception.