F luoride very simply makes the enamel of your teeth stronger - harder. If it is harder, then it will be less likely to breakdown to decay.
Most cities have fluoride in their water supply.
The amount of fluoride present in water is usually about 1 part per billion. If you are not certain if there is fluoride in your water, you should call your water company supplying you - they will be able to tell you if and at what amount there is fluoride in your water. You may also send a sample of your water to be tested. After the approximate age of 14 years - there is no benefit to us from fluoride in our water. The only method of gaining benefits of fluoride after the age of 14 is to topically apply it to our teeth.