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Spot-metering basically means that you can tell the camera to set the exposure based on a very small area in the center (or other region you specify) of the viewfinder. It can be as small as one percent of the total viewing area. This is great for situations where the face is lighted differently from everything around it (which happens most of the time, for me.)
For example, if my subject is in a dim room with a nice strong light on the face I want to set the camera to expose precisely for the face, and not let the camera average out the light on everything. That would produce an over-exposed (too bright) face.
Or, if my subject was outside with sky behind them, again I'd want to use spot metering so that the face would be correctly exposed. Otherwise the camera would average everything and darken it all down, since the sky is so bright. Then the face would come out too dark.
Instead of spot-metering the Canon Digital Rebel used something that I think they called "center weighted" exposure or something like that. It's my understanding that the camera would tend to adjust the exposure for what was centered in the viewfinder but would still try and average things out over the whole viewfinder, with a bit of exposure adjustment for what was in the center region of the viewfinder.
With true spot metering the camera completely disregards whatever is outside the "spot" and doesn't try to "average" what's around it when figuring out the exposure. So the face is always correctly exposed, if done properly.
Later, if I want to get a nice exposure of the sky or the dim background to see details in the shadows I can center my image on those areas and take another shot. Or I can turn spot metering off and get an average overall exposure. For all portrait sessions I take many shots with all different exposures. The most important shots for me though are the ones that are spot-metered with a perfect exposure for the face. The rest of the exposures are just used for supplemental information.
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