Light Meters

Light meters are found inside your camera. Older meters are usually "center-weighted", meaning they read the amount of light within the center of the frame. Newer cameras usually offer more complex meters, allowing them to read the amount of light from many places within the frame. There is usually a box or circle in the viewfinder that designates the metering area. As the light changes within your metering area, your camera will give you some kind of indication of that change.
Meters are very simple tools. They cannot see color. Meters only read tonality, the darkness or lightness of an image. The meter looks at the light in the viewfinder and recommends an exposure by comparing the light pattern and quality with some kind of reference. Meters read the amount of light in and average it. The meter looks at the brightest spots and the darkest spots and says, "Let's be safe and put it in the middle.
This is fine, if you are after "average" toned images. Photographing dark black rocks, instead of getting black rocks, you would get gray rocks, as the meter would average the light to capture gray rocks. Photographing white snow, your film would come back as gray. The meter can't think "Oh, this is snow, so make it white." It can only "average". Understanding how the meter works is very important when photographing extreme or unusual lighting conditions. Let's explore it just a bit further.
There are four variables which can affect your camera's meter: the aperture on the lens, the shutter speed, the quantity of light available at the time you take your picture, and the brightness or tonality of your subject. Your camera already knows the aperture and shutter speed because you either set these manually, or you let the camera choose them. That leaves the quantity of light and the tonality of your subject - and your camera meter has no way to tell the difference between the two. If the sun goes behind a cloud, the quantity of light decreases, and your meter will register that change. In the same way, point your camera at a light colored rock, then point it at a dark rock, and the meter will tell you that the exposure has changed, even though the light has not.
Camera designers had to make a choice. What they chose was to calibrate the meter to accurately expose "middle-toned" subjects, those subjects neither light nor dark, but somewhere in between. The reference point they chose was 18 percent, roughly three stops brighter than black and three stops darker than white. A subject that reflects 18 percent of the light that strikes it is called a middle-toned subject. Probably the hardest technical problem photographers have to overcome is learning how to use their meter to make a properly exposed photograph. Since your camera's meter is calibrated to properly expose middle-toned subjects, it stands to reason that any subject that is not middle-toned will not be properly exposed. Here is the catch: Your meter does not tell you how to make a properly exposed photograph. It tells you how to make the thing you are metering come out middle-toned. If your subject is middle-toned in real life, then your exposure will be perfect. But unless we override our camera's meter, dark subjects and light subjects will also come out middle-toned. And that's not right.

Going Against the Meter
So how do we fix the problem? If we meter a light subject, like a snow-covered field, we know what the meter is going to do. It’s going to make the snow look middle-toned, or gray. We don't want gray snow, so we have to manually compensate by adding back in the light that the meter has told the camera to remove. We add extra light to the exposure of lightcolored subjects to make them appear normal in the photograph. And we do this by either opening the aperture or by slowing the shutter speed from the value that our meter tells us.
In the same way, suppose we are photographing a dark subject like wet tree bark or a dark rock. Our meter is going to try to make the bark look middle-toned. It tries to make everything we point it at look middle-toned. So we have to manually compensate by taking away the extra light that the meter has told the camera to add. We remove light (or add darkness) from the exposure of dark-colored subjects to make them appear normal in the photograph. And we do this by either closing down the aperture or by increasing the shutter speed from the value that our meter tells us.
Let's go over that again. Meter your subject. Compensate from what your meter tells you by adding more light for light-colored subjects or by subtracting light for dark-colored subjects. Take the picture. A handy phrase to help us remember this is: Add light to light, add dark to dark.