Spot metering is a type of metering system in photography which takes into account only about 1-5% of the area that you can see through the viewfinder. Usually, when you focus using the single point AF you can limit your metering to that point and the immediate vicinity. That is often only 1 – 2% of the frame.
The need to use spot metering comes from the fact that sometimes you need to limit your focus only on a small area of the image for balancing an exposure.
An example is a variable lighting situation where there are patches of light and dark areas in the scene. You can meter by placing the AF point on a neutral subject and then meter based on that. Spot metering mode is the closest to getting a level of accuracy that is generally obtained when using external light meters.