Jetting a bike means changing the size of the apertures the gas flows through. Doing this changes the ratio of fuel-to-air mixture. Generally, 15 to 1 is a start for gasoline engines. From there, a LOT of variables creep in. The art of "jetting" means to adjust this ratio so your engine pulls well from idle through full throttle. Simple way is move the taper up, or down, making the mixture slightly richer, or leaner, using washers under the carb needle. If the jet (hole) stays the same, moving the taper up or down will either restrict the hole(leaner), or open up the hole(richer) for a given slider(air) setting. Thus, the ratio changes. Pulling the plugs proves this. A light brown color is best. If plugs are whitish, or gray, the mixture is lean. If plugs are dark brown, or blackish, rich.
Here are few reasons to rejet your bike even if it is stock. Stock bikes come jetted lean because of emission standards. By correcting the stock jetting of your bike, the combustion should be more efficient. The increase in power, although slight, means your bike does not have to work as hard. More importantly, a richer mixture causes your engine to burn cooler, extending engine life. It should also crank easier because the initial coolness of the cylinders displaces some of the fuel in the combustion chamber. This is reason choke is needed until the bike warms up. Rejetting a stock bike requires minimal increase in jet sizes so do not expect noticeable power unless you change your filter and or exhaust. You will then, of course, require more jetting.
Courtesy: Pablo's Page (http://pantablo500.tripod.com/pablosgs500page/id7.html)