What is a Light Meter?

A light meter is an instrument inside your camera that tells you if the amount of light reaching the film will be enough or too much to properly expose your image. The light meter takes into account your shutter speed, film speed, and aperture settings.

How To Read the Meter

Your camera's light meter shows you the amount of light in aperture settings. Each dot on the meter represents a partial "stop", or aperture increment. The numbers represent whole stops. Even though your F-Stop setting controls the aperture itself, the meter readings will also change if you change your film speed or shutter speed.

When to Overexpose and Underexpose Your Images

Even though most of the time you will want to keep your meter reading in the center, there are times you will need to deliberately overexpose or underexpose your images slightly. Sometimes this is for artistic effect but usually it is because you are shooting in conditions that can confused the meter.

Examples of times to overexpose
-Subject is very dark in comparison to background
-Snow
-On a bright day if your subject is in shadow

Examples of times to underexpose
 -Subject is very light in comparison to background
-To achieve a silhouette effect
-On a overcast day to increase color saturation

Source : http://photography.about.com