Fluoride is a naturally occurring ion that comes from Fluorine.
The benefits of Fluoride where discovered when research was done into why it was that people who had naturally 1ppm fluoridated water were found to have much healthier and stronger teeth than everyone else.
Since this early discovery there have been over 30,000 studies into the effects and safety of fluoride and no evidence has ever been found been found that water fluoridation causes any health side effects.
The reason your Dentist likes fluoride is because it has been shown to reduce the number of cavities a person will develop over their lifetime by around half, by making your tooth enamel resistant to the acid plaque bacteria produce.