You have a filter or other lens accessory which
screws into a 58mm thread, but your lens is a 55mm thread. These rings provide
a female thread which accepts your filter or accessory, and a male thread
which screws into your lens. This allows you to mix filters and other
accessories on various lenses which may have different size accessory threaded
sizes. There are a wide variety of different sizes.
For more information about, Cokin 58-55mm Step-Down ring lens to filter .