Stannous fluoride is used in toothpastes and dental rinses to protect tooth enamel from attack by bacteria (cavities, also known as caries). It was the first fluoride used for that purpose, in the toothpaste Crest.
Stannous fluoride can be used with abrasives that contain calcium, which prevents sodium fluoride from being effective. Sodium monofluorophosphate was developed to avoid infinging on the Crest patent.
Fluorides work in two ways. They reduce the ability of bacteria to make acids, and they remineralize the areas of the tooth that have been attacked by acids from bacteria.