You've received some good advice already and some good links where you can read up but I thought I'd throw in another comment.
Try looking around for information on the "exposure triangle". Basically what it means is that there are three factors that affect the exposure of your image, shutter speed, aperture and film sensitivity (or in the case of digital cameras, the ISO setting).
In essence, to maintain the same exposure (I'm not talking about depth of field etc, just the amount by which the image is exposed), you need to double one of these values if you half either of the other. For example, if you half the shutter speed, you need to double the aperture (or double the ISO). Another example, if you double the aperture, you need to half the shutter speed (or half the ISO).
Fortunately, just about all cameras have these three settings configurable in multiples of themselves. What I mean by that is that you can set the ISO value to 100, 200, 400, 800 etc. You see how each value is double the previous value? The same goes for shutter speed and the same goes for shutter speed and aperture.
With aperture it might not be obvious but f/2.8 really is double f/4. That is because the f-number denotes the diameter size of the aperture and an aperture that is twice the size of another would not have twice the diameter of the other. I can explain using maths but I think you get the idea.
Anyways, lots of rambling, I just wanted to point out that there are three parameters that determine the level of exposure (shutter speed, aperture, ISO) and a doubling of one necessitates a halving of any other one in order to maintain constant exposure.
Hope that helps.
? That's the reason I like this mode on my D80.
