Anatomically speaking, the tooth has two parts; the root below the gum line and the crown above the gum line. The crown is covered with enamel and inside the tooth is dentine.
Although enamel is a hard protector of teeth, it can chip and crack as it also erodes from daily friction, grinding, plus the exposure to sugars, soft drink consumption (high levels of phosphoric and citric acids) gastric acids, nicotene from smoking, and other abrasive agents. These cracks gradually fill with stains and debris. As a result, the teeth eventually develop a dull, yellow appearance. They also become more translucent allowing the inner dentine color to show through. Whitening toothpastes and mouth rinses may remove superficial stains on your teeth, but only teeth bleaching agents can remove the stains in the inner layer of your teeth by penetrating and oxidizing stains out of the enamel cracks and dentine.
Oxygen molecules from Carbamide Peroxide agents react with the discolored molecules in your teeth, breaking the bonds that hold them together. The oxygen molecules spread, removing stains and whitening the entire tooth, leaving you with a whiter healthier smile.