During a periodontal exam, your dentist checks the health of your gums. When gum disease is present at the top of the gum line, the tooth becomes separated from the gums. The pockets that separate them contain bacteria and indicate gum disease.
A periodontal probe placed gently under the gums of that tooth will measure the depth. Any pocket greater than 3 millimeters contains significant bacterial buildup. At this point, the gum will start to pull away from the tooth. If left undetected and untreated, it would eventually lead to the loss of that tooth and perhaps bone damage as well.