Technically speaking, when you take a picture, you are simply recording light on film or on a digital sensor. So it's important to "see the light" and understand its effect before you snap the shutter.
The first thing to consider about light is that our eyes see a greater dynamic range (or f-stop range) than consumer print films and sensors in digital cameras.
We can see a dynamic range of about 11 stops. That's why we can see details in shadow areas and light areas at the same time.
Consumer films have a dynamic range of about five to seven stops. Slide films have an even smaller dynamic range of about three stops. Digital sensors can see about five stops.
Keeping all this stuff about dynamic range in mind, it is easy to understand that film and digital cameras don't see what we see when it comes to lighting. That's one reason why people are disappointed when areas in their pictures turn out darker or lighter than they anticipated.
So "seeing the light" is the first step to getting a good exposure. The next step is doing something about it.
When there is a big difference between the dark and light areas of a scene, that contrast range should be reduced. Here are a few ways to do that:
Select a smaller area of the scene with less contrast.
Use a flash to fill in the shadow areas.
Use graduated filters, which are dark on the top and clear on the bottom, to even the contrast range in a landscape scene with a bright sky and a darker foreground.
Use digital darkroom programs to simulate virtually exactly what was seen when the picture was taken even if the original slide, print or digital file has a wide contrast range.
Understanding how shadows affect a scene is also important. Shadows add a sense of depth and dimension to pictures.
So the next time your pictures come back with overexposed or underexposed areas, think about seeing the light. And think about the fact that light is a good teacher. Understand how it affects a picture and you'll get much better results.