There is something called a gingival sulcus that my dentist measures every time I go in for a cleaning. It's the space between my teeth and gums, and the larger that it becomes, the more at risk I am of getting periodontal disease.
When this gap widens, it leaves space for bacteria to get in and cause problems. Since I have pretty good hygiene, my gap is pretty small right now. I floss at least twice a day and brush after every time that I eat, and this helps keep the sulcus from growing larger.