This is where your tooth/teeth present(s) with a cavity or a hole. The cavity has bacteria which will need to be excavated before restoring with a filling either a silver filling (amalgam) or a white filling (composite)

Tooth Coloured Fillings   

In the past, cavities could only be treated with unsightly metal fillings that are alloys of silver and mercury. These fillings, especially when close to the front of the mouth, are highly noticeable and not aesthetic. Modern dentistry has increasingly turned to tooth-colored or composite fillings as a strong, safe and more natural looking alternative. Composite fillings utilize a soft white plastic substance that is hardened with a blue light. These fillings are ideal for initial cavities and those where the majority of the tooth remains intact.