A tooth requires filling when it develops a cavity as a result of decay (also known as caries), or when it becomes worn down, cracked, or damaged.
First, your dentist will numb a small area of your mouth so you will be comfortable while they work. Next, they will remove all the decay, before building your tooth back up with a filling material. Once they have filled the hole, they will smooth and polish the tooth.
A tooth can be filled with different materials. Amalgam is the silvery material that most people think of when talking about fillings, but other materials called composites are also available that will match the colour of your tooth and make the filling less obvious.