Your teeth have pores and act like a sponge; absorbing the colors of the food and drink you ingest each day. Over time, those stains continue to get deeper into the tooth and can no longer be removed by brushing or over-the-counter products like whitening strips. To remove these stains, a professional treatment that ensure that every angle of every tooth stays in continuous contact with the teeth that are stained. The whitening gel in a custom-fitted whitening tray gently opens up the pores and soaks into the tooth. Active ingredients lift the stains to the surface. After whitening, your pores require time to naturally rehydrate and remineralize (ie: pores go back to smaller size). This is why it is best to whiten before bed and apply desensitizing gel after each whitening session. Reducing the amount of colored food (especially coffee, tea, wine, and tobacco) for as long as possible will greatly reduce the chances of your teeth restaining quickly.
In depth look at how teeth whitening works