IF (and that is a big if) I understood your Q right, here's some info.
Exposure meter.. well, it meters exposure
.
It sees the light coming in through the lens and calculates the aperture and shutter speed to get a properly exposed picture.
The problem is, while it's smart, it's not an artist, it assumes that most likely you're shooting some standard scene, and it should be exposed accordingly (google "18% grey" for more info).
While it works for majority of photos, there are situations when you know it's going to be fooled, examples being: there's omething bright in the scene, so the meter will try to "dial it down", as a result you'll get underexposured photo, or the majority of the scene is dark, in this case the meter will try to let more light in and it will result in overexposured picture. Technically, it's neither, the meter just tries to get the overall light to the point, where it's all 18% grey.
Obviously, this is not always the desired "course of action"; this is where the exposure compensation comes in.
Exposure compensation allows you "shift" the meter's "middle point". It is like saying: ok, meter, I know you'll do your best, but I know that dark scene will confuse you, so let me dial it down for you, and you do the rest.
Or, if you know that a bright subject will fool the meter into letting less light in (still - technically it will be properly exposed photo, just not the way you'd like), you will adjust the compensation, so the meter will now operate as you desire.
General rule of thumb: add light to light and dark to dark, i.e., if the subject is very bright, adjust the exposure compensation to + side - so it will allow to properly expose the picture.
An example: a snowy field in a sunny day. if you leave the camera to its own devices, it will carefully dial the exposure down, and you'll get nice, grayish snow. To get proper, bright snow, you'd need to adjust the compensation to +1.5, maybe +2 stops. In essence, you'll be telling your exposure meter: "I know, what you're thinking, but I know what I want to get, so act accordingly".
Opposite is also true: if you want to take a photo of a blackboard, the camera will try its best to get as much light as possible, to be as close to 18% grey as possible. Result: washed out photo of a blackboard. In this situation, you'd need to adjust the compensation to -1 -2 stops.
This is mostly true for center weighted metering, and for earlier cameras - for matrix metering as well. Modern cameras are "smarter", but still, it's better to know your gear, it can only do so much. the rest is your artistic vision. Spot metering will take only the very middle (or, in a more sophisticated case - the area around your focus point) of the picture into account, but the general principle still stands.
Last edited by karro; 06-23-2014 at 08:17 PM.