Plaque left on the teeth hardens into tartar, and from then on a toothbrush is no longer enough. A professional cleaning removes tartar and puts your mouth back in a state you can keep clean at home.
For most people it's every six months. People whose gums get inflamed easily, who wear a brace or who smoke usually need it more often: we set the right interval once we've seen you.

It depends on what you need, and you know before we start: after the visit you get a written estimate, item by item. If there's more than one option, you get them all, each with its cost.