Bacteria in plaque convert sugar into acids. These acids attack teeth. They leach minerals from the tooth substance. This is called demineralisation. If this process continues unchecked, caries develops, a hole in the tooth.
Fluoride supports mineralisation. Dissolved minerals can be stored more easily. Fluoride affects the metabolism of bacteria. Bacteria produce less acid. With fluoride, minerals are better bonded to the tooth enamel. They are less likely to be released from the tooth substance during an acid attack.