The acronym PSU stands for power supply unit, a device that converts AC voltage to regulated low-voltage DC power. This device is part of all personal computers and many other electronics.
In a desktop computer, the power supply unit is a black or grey box with a cooling fan and a bundle of wires protruding from it. Desktop PCs use PSUs that are 12 volts, 5 volts or 3.3 volts. These wires provide the correct DC voltage or voltages to the computer circuits. Different connector shapes prevent incorrect connections. PSUs are not user-serviceable and should be replaced if found to be faulty.