Precipitate Fluoroappetite. It absorbs calcium & phosphate from saliva and makes insoluble precipitate. This will replace the soluble salts of manganese & carbohydrates- what increase the absorption of F in the body; which in fact was already taken out by the bacterial demineralization. So this in fact makes enamel more acid resistance. Not hard per say.
Primary mechanism:- Fluoride ion is easily exchanged for hydroxyl ion (OH ion) coz its smaller than hydroxyl. Plus it has affinity for hydroxyappetite this the fluoroappetite transformation. Voila !
It reduces the rate of enamel solubility

Remineralization, of incipient, non-cavitated carious lesion. That’s why if caries hasn’t crossed the CEJ, you can go for F application, coz it can induce remineralization.

Antimicrobial activity. The function of F releasing GIC. Basically in low conc, it has the ability to reduce microbial adhesion, slow down the ecological succession, by preventing storage of carbs.
Fluoride inhibits bacterial intercellular polysaccharide formation