The ADA (American Dental Association) describes a full mouth debridement as the removal of excessive amounts of plaque and calculus/tartar that interferes with the ability of the dentist to perform a comprehensive and accurate oral evaluation.
Think of it as a first step before a proper dental exam. Technically, this is not a cleaning, but is considered a preliminary procedure to discover if there is a need for additional treatment like filling cavities or doing root canals. It is considered a definitive treatment for gingivitis (gum inflammation) or a pre-surgical treatment if the disease has progressed to severe periodontal disease.
Plaque is a soft, sticky film that forms on teeth after eating. It is composed of bacteria and bacterial by-products, regardless of what foods are eaten. The bacteria irritate the gums, which means they may bleed a bit. If not removed by regular brushing and flossing, the plaque hardens into calculus (a.k.a. tartar), a deposit somewhat like cement or lime deposits on your plumbing, that is formed from the plaque in the mouth and the minerals in saliva, and that is bad news for your teeth and gums. Unlike plaque, calculus cannot be removed by regular brushing and can only be removed by your dentist or hygienist with dental instruments.