The idea is to meter from something that is neither an extreme highlight nor an extreme shadow. Camera reflected-light meters tend to place what they measure near a middle tone, so if you meter from something very bright or very dark, the exposure can be pushed in the wrong direction.

'Brother Backlit Sky' works because the sky off to the side of the sun is often a reasonably moderate reference compared with the sun itself, deep shadows, or very dark foreground elements. It gives you a consistent baseline that is less likely to fool the meter than metering directly from the brightest or darkest parts of the scene.

In other words, the book’s 'Brothers' are simple metering shortcuts: use a part of the scene that’s closer to the middle, and you’ll usually get a pleasing starting exposure. As you gain experience, you move beyond the shortcut and choose what to meter based on what matters most in the image and which tones you want to preserve.