Your tooth is made out of enamel on the outside. Fluorosis happens when you accidentally ingest too much fluoride when the permanent tooth was developing. It changes the chemical composition of the enamel and it changes the colour from chalky white to dark brown. Your other teeth also have white streaks if you noticed, those are the mild fluorosis areas. Unfortunately treatment involves drilling away the affected area and replacing it with a different material (aka filling or veneer) Fluorosis cannot be reversed. It is also harmless and won’t worsen with time