Power Good Signal (or Power_OK, or Power_Good) is a signal that tells the motherboard that the PSU is ready for use. When the PSU starts up, it takes some time to "warm up" and start generating the right DC voltages that computer needs. You don't want anything could go wrong if computer were allowed to try to boot up while PSU is not powering correctly. To prevent this, PSU puts out power good signal after it completes its internal tests and determine the power is ready for us. The motherboard will not boot until there is power good signal.