An 80 PLUS® power supply (PSU) is certified to convert at least 80% of AC power from the wall into usable DC power for your computer, with higher tiers (Bronze, Silver, Gold, Platinum, Titanium) indicating greater efficiency (82% to 94%+) and less wasted energy as heat, meaning lower electricity bills and better component lifespan. It's a crucial certification for PC building, showing a PSU's ability to deliver stable power efficiently under different loads.