Tartar results from plaque that is formed on teeth by bacteria which reside in our mouth. The proteins and other byproducts of what we eat are used by these bacteria to create a sticky substance that creates a layer on the teeth. Plaque is most likely to form in areas within the mouth that are difficult to reach.
Tartar is formed when the plaque hardens. It is a lot tougher to remove tartar than plaque as tartar contains minerals that make is hard and more adherent to the teeth.
Bacteria find tartar a very easy place to hide and cause more damage. They release acids from food that is eaten which lead to cavities. Infections and even loss of teeth can occur if the tartar is not taken care of, ideally by visiting a dentist.