As damaging as plaque can be, it is easily removed by mechanical brushing. Plaque deposits build up on the teeth fairly quickly after eating and drinking, and if they are not brushed away at least twice a day, they can lead to tooth decay and periodontal disease. It takes just one day for bacteria to build up enough to make your mouth susceptible to disease.
If plaque is not removed, it hardens and becomes calculus (tartar). Calculus cannot be removed with regular brushing. Your hygienist has special instruments designed to remove tartar.