Fluoride is one of the best discoveries in modern dentistry. It promotes re-mineralization of the tooth structure, and helps the prevent cavities and de-mineralization.
Cavities occur by the bacteria on the tooth metabolizing the sugars in your diet. Their by acidic by-products de-mineralize the tooth, and the tooth structure eventually collapses or 'cavitates.'
Fluoride works by uptake into the outer enamel shell of the tooth. When a tooth re-mineralized, if fluoride is on the tooth, it will form inside of the enamel crystalline structure. This forms a much harder shell on the tooth which resists cavities.