Stops.. a stop is just a measurement of light really. A stop can be increased or decreased by a wide or closed aperture or a slow or fast shutter speed. So when I say camera sensors can capture around a 7 to 8 stop range, it's really zones I suppose. Imagine zones being broken into different levels of tone - black being zone 1, an average colour (a middle grey, or a brick wall or blue sky on an averagely sunny day) being zone 5 and white being zone 10.
So if your scene contained a completely solid black element and a completely solid white element, either your black element would have to go dark grey to capture the white element - because the sensor captures too much light from the scene to keep the black element black or the white element would have to go light grey to capture the black element because the camera can't capture enough of the light reflected off the white element to keep the white element bright white. You'd have to decide which is the most important.
So if you wanted to capture a black cat who is sat on a field of snow, to expose your cat properly, you'd have to accept the snow is going to have to go a little grey because the sensor doesn't capture enough of the light reflected from the snow to make it bright white.
Or in another example, a sunset - because the sky is really bright as the sun sets on the horizon and your foreground is going to be dark, you wouldn't be able to capture all the detail of the foreground as well as getting all the colours in the sky (you can with graduated filters but that's another topic) so you'll often see sunset images being silhouettes with nicely exposed skies.
For example (a random creative commons image from flickr)
The dynamic range here is too wide to get detail of the surfer who in the available light is going to be really dark and would need the shutter being open for a long time or the aperture to be opened up wide, and the sky which still has really bright tones. You'd have to decide if you wanted to capture the detail of the surfer - and have a really, really bright sky - or capture the colours of the sky - and have the surfer being a silhouette as we see. Your camera won't get both.
Hope this explains it!
Oh as regards to spot metering, I believe (I could be wrong for canon cameras) the camera meters the spot your focus square is aimed at rather than the centre of the viewfinder. So if you use your directional pad to focus on something on the left of your viewfinder, the light meter in your camera will read off that spot. Certainly this is how it works on Nikon.
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