The depth of the space between the visible top edge of the gum line and the point to where the gum attaches to the actual tooth is what is being measured.
As the doctor or hygienist poke around your teeth with a measuring tool called a probe, you will hear the doctor or hygienist say a number that ranges from 0 to 15, also known as the pocket depth. The doctor or hygienist will measure each tooth in six different spots. Any number between 0 and 3 will indicate that your gums are healthy.
Between 3 and 5 indicates gingivitis. Generally, a routine preventive cleaning will remove the elements (typically tartar and plaque) which typically cause inflammation in the gums.
Anything more than a 5 means periodontal disease, aka gum disease. Gum therapy is prescribed after a 5, which is different than a routine preventive cleaning.
Surprise your hygienist at your next visit. And ask, what is my gum number?