Your mouth has its own unique microbiome filled with healthy bacteria that help break down food, deliver nutrients to the rest of your body, and protect you from foreign agents. When food is leftover in your mouth for too long, it can lead to the growth of unwanted bacteria.
If left alone, this bacteria can form into a sticky, film-like substance called plaque, and eventually turn into tartar (calculus). While plaque may be more difficult to remove, tartar cannot be removed without professional dental tools.
The bacteria in the plaque and tartar eat away at the enamel of your teeth and cause the hollow spaces we know as cavities.