Fillings are materials placed inside the tooth to replace the part of the tooth structure which has been lost because of the process of cavitation (decay) or damage.
A cavity forms in a tooth as a result of the action of bacteria in plaque. Plaque breaks down carbohydrates, especially sugars, in food to form acid. Over a period of time this acid erodes the enamel, which results in cavity formation.
In the very early stages of the cavity formation, a filling may be avoided by diligent home care measures and regular check ups by your dentist. More advanced stages of cavity require a filling to replace the missing tooth structure.
There are various materials available for fillings. Each material has properties that make it more suitable in a situation compared to the other material. Your dentist can help you choose what material would work best for you as well as making sure you know the cost associated with it.