If plaque and tartar is left on the teeth it provides the right conditions for bacteria to thrive. The bacteria irritate the gums, which can cause them to bleed more easily. You may notice this if you are brushing your teeth or eating, and your gums bleed a bit. This is the early stage of gum disease called gingivitis.
If you have gingivitis, your dentist or hygienist will clean your teeth by scaling and polishing them. They may also recommend an antiseptic mouthwash containing chlorhexidine and show you how to brush and floss your teeth effectively. Most adults have some degree of gum disease.