The periodontal tissues, also called the periodontum, are specialized tissues that surround the teeth and support them in the jaw bones (the maxillary and mandibular bones). These tissues include the gingiva (the gums), the periodontal ligament, cementum (that covers the roots of the teeth), and alveolar bone, which is the thick ridge of bone in your jaw that holds the teeth.  Periodontal disease is the most common cause of tooth loss in adults.
Multiple species of bacteria are always present in the mouth. Plaque is a sticky film on your teeth (and sometimes under your gums) that bacteria attach to. These bacteria produce acids after you eat or drink anything and these acids will eat away at your healthy gum tissue, your tooth enamel, and can cause cavities in your teeth. If you don’t remove plaque regularly (both at home and at your dentist’s office) it hardens and becomes a substance called tartar.
As plaque and tartar build up, they can cause the gums to separate from around the tooth, making pockets under the gum line. These pockets fill with bacteria, infecting the gums and are very hard to clean at home. The inflammation caused by the bacterial infection also affects the bone, causing it to start disappearing (resorbing). When this happens, the tooth can become loose and fall out, or need to be pulled out.
Consider adding a sentence here about gum infections and the implications of them if left untreated?