Dear Confused and Fillingless,
I understand the confusion and I am so glad you asked!
First, a cavity is a hole in your tooth. When bacteria is left on a tooth for a long time and fed sugar, the bacteria secrete acid. The acid actually dissolves the tooth, creating a hole in the tooth. It is then easier for bacteria to hide in the hole and continue to secrete acid, making the hole deeper and/or wider.
The outside of the tooth, the part you see and that everyone wants very white, is called enamel. It is very hard and can resist acid for a long time. It can also be healed with fluoride. Fluoride bonds with crystals in the enamel and makes the crystals grow. The crystals fill the hole, making it disappear or “heal”.
The enamel is much like the peel on an orange. It is tough and resilient, but it covers a softer and weaker part of the tooth called the dentin.
Once bacteria has dissolved the enamel and penetrated the dentin, the tooth must have a filling. The dentin dissolves easily and the hole will grow quickly.
This is why we take xrays every year. A small cavity in the enamel can sit there a long time, but a cavity in the dentin can get big and cause problems.
I hope this information helps you. It sounds like your dentist is conservative and wants to try fluoride before he does any surgery to the tooth ( ie the filling).
Be sure and brush 2-3 times per day and floss every night before bed to remove the bacteria. Then use the fluoride as your doctor directed.