It is an efficiency rating or basically how much more it will pull from the wall than its rated output.
Example 500watt 80% efficient will pull 625watts from the wall at full load! at 90% efficiency the same PSU would do 555 watts from the wall!
90+ is a bogus marketing term, 80 Plus is run by Ecova and gives out certifications to power supplies based on the efficiency they manage at 20, 50, and 100% load.
Recently there became 6 tiers to it, from basic through gold, platinum, and titanium.
For an end user, the price difference between a Bronze and a Gold unit will almost never pay for itself in energy savings over the life of the unit so don't go and buy an 80+ titanium unit thinking it will pay for the price difference, it never will. 80+ became a thing because it is very useful to the power companies(less capacity required) and is useful for corporations. In a large office that requires AC you pay for all the power your computer uses twice, once to power the computer, and a second time to run the AC to remove that heat from the building, for massive call centers running at 90% instead of 80% can make a big difference in overall facility power consumption in the summer.
I have a post about PSU efficiency myths, you can find it in my signature.
The important thing to know is efficiency is based on a curve. You want to accurately determine what load you will likely be using in your rig and try to match a supply that benefits the efficiency curve. Power supplies lose efficiency as the load drops and typical efficiency ratings vary between 50-70% loads. Extra energy not used by the supply is dissipated as heat. Less heat equals a longer life and a quieter running supply.
The curve argument is bogus and just results in people grossly oversizing their PSU. You should oversize a bit to give it some headroom to allow for component aging, but sizing to be at 50% is just silly. Efficiency tends to only drop by about 2 percentage points from 50% load to 100% load. A few years back i plotted this with an honest Y-axis, not zoomed in like most people do.
Power supplies are the devices that power computer, servers and data center devices. They convert AC power from electric utilities into DC power used in most electronics. The 80 PLUS® performance specification requires power supplies in computers and servers to be 80% or greater energy efficient at 10, 20, 50 and 100% of rated load with a true power factor of 0.9 or greater. This makes an 80 PLUS certified power supply substantially more efficient than typical power supplies.
The farthest left data point is the 10% load that JonnyGuru collects in their reviews, no one measures below that, though the 5VSB which is always on has become rather efficient in recent years, it used to be ~70%, now they are nearly 80% at 4A which is quite good for a cheap little circuit. http://www.jonnyguru.com/modules.php?name=NDReviews&op=...
The reason no one pays a ton of attention to super low load levels is because it doesn't matter. Lets take a system that is 50W at idle and 500W at load. The difference between having a PSU that is 85% vs 75% efficient at 50W is only 7.86W/hour, the difference between having a PSU that is 90% vs 88% efficient at 500W is 12.63W so the 2% efficiency difference at load results in 60% more waste heat than the 10% efficiency difference at idle, much larger power savings gains can be had by improving efficiency where there is a significant quantity of power in play.