Our teeth are continually covered in saliva which contains calcium and other substances which help strengthen and protect the teeth. While this is a good thing, it also means that we are prone to a build-up of calcium deposits on the teeth. This chalky substance will eventually settle over time, like limescale in a pipe or kettle. Typically, it is tooth colored and can easily be mistaken as part of the teeth, but it also can vary from brown to black in color.
If the scale or tartar can accumulate on the teeth it will, unfortunately, create conditions for bacteria to thrive next to the gums. The purpose of the cleaning and polishing is basically to leave the surfaces of the teeth clean and smooth so that bacteria are unable to stick to them and you have a better chance of keeping the teeth clean during your regular home care.