When the pH on the tooth surface becomes acidic, the enamel starts to demineralize so your tooth is more susceptible to bacteria. Fluoride in the mouth enhances remineralization of the enamel using the calcium and phosphate ions in the saliva. There's different sources of oral fluoride. Stannous fluoride in certain toothpaste gels sticks to the enamel surface. It's supposed to be stop bacterial proliferation > biofilm > plaque and kill bacteria. Sodium fluoride works better in a toothpaste because it is more abrasive.

In Europe there's biomimetic hydroxyapatite to create a new layer around teeth to harden the existing enamel so fluoride or lack thereof isn't as big a deal.