There are two ways hat fluoride helps our teeth, one happens when we’re kids before our teeth are in our mouth, the other happens after our teeth are up in our mouths.
When as kids we’re exposed to fluoride it can be absorbed into our teeth as they are developing. This makes for harder teeth as they erupt and can gives us a lifetime of benefit and a lowered risk of tooth decay.
Once our teeth are in our mouths they are exposed to acids which can dissolve the minerals in the enamel. The fluoride helps to re-strengthen the enamel by helping the mineral structure re-form.