It means the conversion factor of yearly yield in kWh to installed PV in kWp. "Yearly sum of solar electricity generated by 1 kWp system". It's not about the physical PV efficiency.

The 75% performance ratio represents system losses not module efficiency. So a system rated at 1kW(peak) which may be 5-6 square meters in size, and which is exposed to annual sunlight of 1200 kWh/m2 might only produce 900 kWh of usable power.

So efficiency has to do with the rated power per surface area and the performance ratio reflects all the system electrical and environmental factors that reduce the amount of usable power below what would be expected based upon module ratings and annual sunlight exposure.