It is another word for a professional cleaning. Regular brushing and flossing is not enough to remove hard plaque and tartar. It gets build up over time and can only be removed by a professional clean. It also, removes teeth staining caused by food, drinks and smoking.
Dentists usually recommend twice a year a professional cleaning. This will prevent tooth decay and gum disease. The frequency of the cleaning depends on how thorough you brush and floss your teeth, how often plaque gets build up in your mouth as well as your risk of infection.
A proper cleaning consists on scaling, polishing and application of fluoride. Scaling is done with a small instrument that goes around your gums and between your teeth. Polishing gets done with a gritty toothpaste. It removes any excess of bacteria leaving teeth surfaces smooth. Finally, the application of fluoride protects teeth against disease and also helps them with any sensitivity.
After the scaling you are left with a nice clean sensation.