Spot metering isn't anything to do with setting white balance - it's a means of establishing correct exposure in difficult situations. Most DSLRs which allow spot metering have a small circular area in the centre of the viewfinder which is either a clear area easily discernable form the rest of the textured screen or defined by a clear black ouitline. That's the area from whcih a spot metering reading is taken - the rest of the screen is ignored by the meter. Typically spot meters in cameras have a spot metering angle of between about 6 and 9 degrees depending on model.
The basic idea is that using your own judgement, you select an area of the scene you wish to photograph which represents an average tone, or a tone which will produce an overall good exposure of the whole scene. Obviously, a basic understanding of exposure principles is needed to make the most of spot metering. Without that understanding spot metering can cause more problems than it solves.
Take a situation where you're photographing a landscape with distant mountains where you judge a tiny part of those mountains to be a tone which will produce a good exposure of the overall scene. Placing your spot reading zone over that area and adjusting your exposure to suit that area is how you would proceed in practice.
Truthfully, the 6 -9 degree spot in your DSLR, while potentially very useful, is a little large compared to a decent hand held meter where the spot reading zone would be as little as one degree in a really decent meter- obviously enabling you to take a spot exposure reading from smaller areas and at a much greater distance for greater accuracy.
Having said that, a spot metering facilty in a DSLR is a very useful feature to have provided that you have a basic understanding of metering and how it works.
Hope that helps.
