If plaque and tartar is left on the teeth, it provides the right conditions for bacteria to thrive. The bacteria irritate the gums, which means that they are inflammed hence tend to bleed easily. You may notice this if you are brushing your teeth, or eating, and sometimes your gums may 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.
If gingivitis not treated and nothing is done about it, the inflammation will work its way down towards the foundations of the tooth causing a "periodontal pocket" leading to Periodontitis. Again, within the confines of the pocket, the conditions are such that the bacteria can cause more damage.
Gum disease can break down the support (bone) structures of the teeth, so that eventually, they will become loose. The problem is that until it gets quite severe, the person often has no symptoms. Sadly, the damage to the support structures of the teeth is irreversible. The good news is that if gum disease is caught in time, its progression can be halted and improved upon, and that is the key.
To stop gum disease from progressing, your dentist may advise periodontal therapy, or deep cleaning. This gets rid of the bacteria in the pocket and provides the necessary conditions for healing to occur.