Hello
It is not cavity, but these are calculus deposits on teeth that have led to receding of gums away from teeth.
Treatment would be deep scaling and curettage (deep cleaning of teeth) followed by oral hygiene and gum care instructions and regular follow ups.
You might require minor gum surgery in case the gums don't heal or remain detached.
Next Steps
Visit your Dentist for the treatment.
Health Tips
Don't ignore it, eventually your teeth will start shaking (mobility)