Teeth form cavities in them when bacteria and food matter stay in contact with your tooth for a period of time that allows a hole to form in your tooth. Often this hole is a darker color, but can start out as a white mark as well.
The only way to prevent the hole to get any larger is to remove the bacteria that are inside the hole, and fill it with a filling material that prevents future leaking, and allows the tooth to continue to function normally. Dr. Bliss primarily places white filling material called composite unless another material is specifically requested.