Light Meter ----- Metering ----- Exposure Bias ----- Thinking Like Your Camera ----- Thinking Beyond Your Camera
Simply put, the light meter in your camera determines how much light is going to look correctly exposed. It then makes decisions based upon that information. In Program Mode, it sets the aperture (fstop) and the shutter speed -how big the window opens and how long it opens.
The light meter can be pin-pointed to the very center of your frame (called Spot Metering), it can consider the entire frame (Matrix metering), or somewhere in between (called center-weighted).
Spot metering is nice because you can point your camera to the very spot you want exposed correctly, press half way down (or lock the exposure) and then compose your shot and fire. Matrix metering considers the whole picture and tries to find an exposure that will produce the bets results.
EXPOSURE BIAS -
Most cameras have an exposure bias.
THINKING LIKE YOUR CAMERA -
In order to correctly expose your image (especially your subject) - you need to think like your camera. Think correctly expose each part of your image... the background, the foreground, the subject, the highlights, the lowlights. You get one image that combines all of that... the trick is to keep it simple enough to be aesthetic.
THINKING BEYOND YOUR CAMERA -
Landscapes and still life allow us to make multiple images taken at different settings (multiple exposures) and then combined them using software -- HDR.
For person shots -portraits, weddings, etc - we don't have that luxury. Here's a short video from a wedding photographer named Cliff Mautner. Here he is talking about using a kicker flash. But note that he talks about using an exposure bias (I think he speaks in terms shooting 1.5 to 2 stops below his ambient light reading).
Finally, thinking beyond your camera. In Cliff's work, he does a lot of work with flash and/or directional lighting. In order to emphasize that single light source, he underexposes the rest of the shot. So he probably meters on something that he wants to underexpose... he has his exposure bias set down a stop or two -- he then sets the fstop and shutter speed manually to match that meter reading (most likely on spot metering or center weighted) -- then he kicks in a flash or has the window light do the highlighting for him. This is called low-key lighting, where the overall image is dark. Notice how your eye is led right to the highlighted areas.
So, I tried this myself one time with limited success: