Hi mam,
This is stains or dark calculus ( deposits) caused by chromogenic organisms and food deposits. It will not be removed by flossing or brushing by your own. You need a professional cleaning called scaling .
Next Steps
Visit the dentist nearby and do a routine examination mam
Health Tips
continue brushing twice daily, gargling after every meal, flossing, cleaning your tongue, regular visit to dentist( twice a year), use correct tooth brush and correct brushing technique mam. Happy for your awareness
Hope this helps.