The exposure meter tells you before taking the photo if it is going to be properly exposed. Typically, is goes from negative three to positive three. Usually, there is a line or a dot that moves from left to right along this scale. Depending on your composition, what you are looking at through your lens, and the settings and lighting around you, this line will move around. A beginner’s goal is to get that line to the center for a balanced exposure. If the line is on the left side of the 0 in the middle, then your photo might be slightly underexposed, if the line is on the right, you may be overexposed.
The numbers of the scale signify how many stops of light are under or over exposed, so using your knowledge of the exposure triangle, you can change your camera settings to move that line to the middle of the scale achieving a good proper exposure. If it is completely under or over exposed, there will be a little arrow to the left or the right.
Is the light meter always right?
It is not always right, depending on how you want your photos to look. Sometimes you want your photo to be a bit bright or maybe you want a photo mostly dark with just a single point of light. In these cases, the camera’s light meter will tell that your photo is going to be over or underexposed. Just like many of the rules of photography, using a camera’s light meter is only a tool, it’s up to you as the artist to use it to help you take better photos.