Plaque is a thin bacterial film that adhere tightly to the teeth. Many a times, plaque can be dealt with regular cleaning and flossing. However, if it sticks around the teeth they may lead way to some
other serious conditions like gum diseases with initial presentation of bleeding gums and destruction of teeth supporting structures in the later and advanced stage.
Unremoved hardened plaque is calculus which is a visible mass of yellow, brown black color. They may get deposited along the gum line as supragingival calculus and below gums as subgingival
You may need the help of a dentist to intervene this stage to remove them from the teeth surface.