Stepping rings screw to the front of your lens so you can add different sized filter accessories to your lens. They are also known as adaptor rings. There are step up rings which let you attach accessories that are one size up and there are step down rings that let you attach one size down accessories. For example, to fit a 52mm filter on a camera lens that has a 49mm thread, you would need to use a 49mm-52mm step up ring. To fit a 55mm filter on a camera that has a 58mm lens thread, you need a 58-55mm step down ring. Stepping rings are available in various sizes and are made to fit assorted lenses.
Some of the rings screw on and some slip on and can be fitted for bayonet type fixtures. There are plastic rings and metal rings and it's easier to use metal with metal and plastic with plastic. Stepping rings range in price from about $20 to a few hundred.