Fillings do just what the name implies — seal a small hole in your tooth, i.e., a cavity, caused by decay. This prevents the decay (a bacteria-induced infection) from spreading further into your tooth and, if untreated, continue on to the sensitive inner pulp (nerve) tissue located in the root canal.
There are a variety of materials used to fill teeth these days, but the process of filling a tooth is similar regardless. The first step is a clinical exam of the tooth with x-rays, to determine the extent of the decay. Then the decayed area of the tooth is removed, usually with a handheld instrument such as a dental drill. Of course, your tooth will be anesthetized first, so you won’t feel any discomfort. After removing the decay, the remaining tooth structure is roughened or “etched” with a mildly acidic solution; then translucent cement is applied to bond the tooth and the filling material together.