A 400W 6-pin power supply refers to a power supply unit (PSU) that provides 400 watts of total power output and includes a 6-pin PCIe connector. The 6-pin PCIe connector is typically used to supply additional power to discrete graphics cards (GPUs) that require more than what the motherboard can provide through the PCIe slot alone. This type of power supply is commonly used in mid-range systems or older PCs with moderate GPU requirements.