Spot Metering is a way to meter a scene while concentrating on a small section of it. Many newer cameras offer spot metering as an option to full scene or partial scene metering. Spot metering allows the photographer to pick a specific area (usually about 5% of the scene) and meter only that area for a very accurate reading.
Spot metering allows you to get the most important part of a scene metered correctly, but can over or under exposure other areas.