Not all lenses are designed with standard filter accessory screw in thread sizes. For these lenses, many of which are very popular, the way to mount a filter is via an adapter ring. A main feature of adapter rings is that you can utilize one size filter with a number of different sized lenses via different adapter rings.
An Adapter ring consists of two parts: an adapter ring and a retaining ring. The non-threaded filter is held in place between the two as you screw the retaining ring into the adapter ring. The adapter ring is than screwed directly into the lens or is mounted to the outside diameter of the lens barrel via a setscrew lock ring adapter.