the background is two stops darker than the key light
Case 1: Incident light
1) You take a measure of the incident light, for example with an incident light exposimeter. First on your background. It says f/5.6
2) You take a measure near your subject, for example a face. It says f/11
This means: "The light over there, at the background is not that much, use a wider f/stop so you can have more light, and over here at the face close your aperture to f/11 because if you do not do so your face will be blowned".
Conclusion 1, the background light is two stops less than the Key light.
Case 2: Graycard
You could measure the exact same thing as the case 1 using not an incident light metter, but using a gray card. Put your card on one spot, and measure it on the camera, and do the same near your subject; main light.
The camera is comfort with this graycard "I will not take a photo of this gray too dark or too light" The camera say.
Case 3: Measure the zones
Cameras are dumb (sorry cameras) so they do not know if what you have in front is a bright sky or a dark soil. All they see is amount of incoming light to the camera, reflected light.
The two previous cases refers to the incoming light and reflected light on a controlled target. But not all objects in the universe are gray... are they? At least not on your picture.
Some you need them darker, like the interior of a farm, or lighter like the skin of a caucasic lady.
If you let the camera decide, it will try to expose whatever has infront to achive a middle gray. The exposure on the face could say f/8 which would be ok for the skin on that situation (a gray card is simmilar to caucasic skin)
But the interior would read f/4, so it is saying, "Whatever dark subject or poorly iluminated place this is, for me to show you as grays you need to expose it at f/4". The camera thinks all objects are gray cards.
When you expose your lady at f/8, your background will be darker... 2 steps darker. Which is not bad, it is simply... darker.
Intrinsec object color.
Imagine the "United colors of Benetton" campain, and imagine that you forgot your incident lightmeter.
If your center model has a darker skin (African model) and you use that measure as middle gray the camera will think, "oh we do not have much light, lets expose more" and your blond model will look like a gohst.
Some skins are one or two stops darker than others.
This is not a racial thing... (probably it was at some time in history) but now days is a coincidence to the middle gray of a graycard.