Over time, teeth can become stained or discolored, especially after smoking, taking certain medications, or consuming foods and beverages such as coffee and tea. Using a chemical process, your dentist can bleach your teeth in one of two ways. He can do an in-office procedure, or provide you with a system to use at home.
Keep in mind, your teeth can become stained again if you continue exposing them to the same substances that originally stained them. Because whitening products are not meant to clean teeth, it is still important to continue practicing daily oral hygiene by brushing twice a day, flossing at least once a day, and rinsing with an antiseptic mouthwash daily.
Source: https://www.webmd.com/oral-health/cosmetic-dentistry#1