Bottle conditioning is the process by which you carbonate the beer you have produced in the bottles they are to be served in. It is a handy way for brewers to carbonate their beer without the need for a kegging system and some sort of bottle filling tool.
Bottle conditioning involves the adding some additional sugar, called priming sugar to your beer, followed by transferring of your beer to bottles and capping them. The amount of sugar you add depends a bit on personal preference, and how quickly you plan on consuming your beer. In a perfect world all original sugars in the wort will have been consumed and the only additional sugars consumed will be the ones you are adding, and they will be consumed quickly so that you can enjoy your beer ASAP, in practice this is rarely the case.