Power good signal
The Power Good signal is a signal provided by a Power supply [unit (computer)|computer power supply unit] to indicate to the motherboard that all of the voltages are within specification and that the system may proceed to boot and operate.
ATX Power Good
The ATX specification defines the Power-Good signal as a +5-volt signal generated in the power supply when it has passed its internal self-tests and the outputs have stabilized. This normally takes between 0.1 and 0.5 seconds after the power supply is switched on. The signal is then sent to the motherboard, where it is received by the processor timer chip that controls the reset line to the processor.The ATX specification requires that the power-good signal go high no sooner than after the power rails have stabilized, and remain high for after loss of AC power, and fall at least before the power rails fall out of specification.
Cheaper and/or lower quality power supplies do not follow the ATX specification of a separate monitoring circuit; they instead wire the power good output to one of the lines. This means the processor will never reset given bad power unless the line drops low enough to turn off the trigger, which could be too low for proper operation.