An adapter or adapter ring (Step-up / Step-down) is a ring with threads on both sides. You put it on the front of the lens of your camera. Almost all camera lenses are threaded to the front. These are standard sizes. With an adapter, you can use a filter, hood, conversion lens or other camera accessory with a different sizes thread. To use a 77mm filter on a lens that has 67mm thread, use a 67-77 mm adapter ring etc.