Grain has a maximum extraction potential. that is, there is a maximum amount of sugar that can be converted from the grain.
I'll randomly pick 1.050 for a hypothetical recipe. This is the absolute maximum that could be squeezed from the grain and a special laboratory congress mash.
You mash and carefully measure the gravity, before the boil starts and after the wort is very well stirred. You chill this sample to 60F and measure it with a hydrometer. You find this to be 1.040.
You achieved 1.040 out of a potential of 1.050. I'll simplify this with 40 out of 50. That is 80% and is pretty good. 40/50*100.
1.040 is the Pre-Boil gravity. When you boil, it concentrates the sugars and increases the final gravity by 5, 10, or 15 points or somewhere between.
Lets say you concentrated 10 points to and increased the gravity to a final of 1.050 after boiling.
Later on, after fermentation is done, you measure gravity again and adjust for the alcohol and CO2. You find this sample is 1.008.
A second efficiency you will be interested is this fermentation/yeast efficiency also referred to as attenuation. The yeast converted 84% of the sugars. 100-(8/50*100).
If you find that you brew the same beer and the yest attenuation is inconsistent but also then you need to focus on your fermentation.