The stains you are noticing on the molar teeth may be surface/extrinsic stains, but they can also sometimes be associated with early decay or changes in the tooth surface. Since you also have tartar (calculus), a proper clinical examination is needed to determine the exact cause.
Next Steps
Schedule a dental examination and professional scaling/cleaning to remove the tartar and surface stains.
After cleaning, the molars can be assessed more clearly for cavities, enamel defects, or deeper staining.
If clinically required, your dentist may advise an X-ray to check for decay between the teeth.
Health Tips
Brush twice daily with a fluoride toothpaste and clean between the teeth regularly.
Avoid trying to scrape tartar or stains yourself, as this can injure the gums or enamel.
Reduce frequent consumption of tea/coffee, tobacco, and other staining substances if applicable.
If you develop pain, sensitivity, swelling, bleeding gums, or a visible hole, please have the tooth examined promptly.