Reflected Light and Algorithms, oh my!
Your in-camera meter measures how much light is reflecting off the scene it is metering. If everything reflected the same amount of light we would be home free. Obviously, a nice polished piece of chrome on a motorcycle will reflect more light than the skin of a pretty girl sitting on the bike.
Since the amount of light being reflected from different parts of the scene varies, in-camera meters measure the light intensity in different parts of image and uses an algorithm to determine the most suitable exposure for the final picture. We get to tell the meter which algorithm to use. Well, the camera manufacturer gives us the chance to select from several metering modes that each represent a different algorithm.