The efficiency certification ( 80 Plus, 80 Plus Bronze, Silver, Gold, and Platinum ) basically refer to how efficient the power supply can run at at a certain load, or what is its maximum efficiency. The best would be 80 Plus Platinum, while Gold comes in second. You can identify their ranking easily just based on the naming convention.

Efficiency also indirectly indicate the quality of the components used in the power supply that was awarded the efficiency certification ( Bronze, Silver or et cetera ) since to achieve a high efficiency you need to have good quality components.