Take a look at this picture:
What you see there is a nice scene of some fishing boats, with a good tonal range, plenty of colour and some nice late-evening sunlight. What your cameraâ€™s lightmeter sees is this:
Try it out for yourself: copy the fishing boat scene onto your hard drive. Start up your image editing software and open both pictures. Lightmeters only see in black and white, so reduce the saturation of the fishing boat shot to zero. Next, add a Gaussian blur set at its maximum level, so that the whole picture is reduced to a field of grey. Use the eyedropper tool to measure the RGB colour value of the resulting tone. You should find that it is a mid-tone grey with an RGB value of around 127,127,127.
Itâ€™s an interesting and curious fact that any average scene reflects 18 per cent of the light falling on it. Look out of your window, and unless you live in Antarctica the scene you see is reflecting exactly the same amount of light as the scene out of my window. That 18 per cent reflection is exactly the same as a mid-tone grey, mid-way between black and white.
Light meters are calibrated with this fact in mind. When your camera takes a light reading, the meter averages the scene and adjusts the exposure to produce that mid-tone grey (or 12 per cent luminance, but thatâ€™s another discussion altogether). If you point the camera at a black stage curtain, it will try to make the black into a mid-tone grey, so it will over-expose. If you point it at snow it will try to make the white into grey, so it will under-expose.