They are mostly just patches of hyper-pigmentation which are very normal. If they burn, itch, ache or you are able to feel them with your tongue, it's best to get it checked by a dentist. Tartar is also a buildup of mouth debris which can be removed by a dentist. It's advised to get this removal (scaling) done once a year, and once in 6 months if you are a smoker
Next Steps
check and see based on the above- mentioned signs