When you don't properly brush, overtime tartar accumulates, this is known as calculus. Which results in irritation of the bone and gum tissues resulting in their infection and inflammation and ultimately loss of these tissues. So you end up losing your healthy bones and your gums begin to slide down. Over time the teeth get loose and fall off. All this destruction can be easily prevented by just brushing regularly and visiting a dentist once or at least once in 2 years and getting a scaling or teeth cleaning procedure done. So please maintain your oral health. Don't neglect it.