In the absence of good brushing and flossing, plaque builds up on the teeth and hardens to form tartar.
Plaque itself is a mixture consisting of bacteria, minerals, and food reminants that can cause irritation and (with time) inflammation of the gums. Plaque quickly hardens to form tartar deposits that can no longer be removed with brushing or flossing.
This can lead to inflammation (gingivitis) and, in the absence of medical attention, periodontal disease.