Stepping rings let you use filters with a thread size different from the thread size of your lens. So, for example, if you have a new lens with a 52mm thread size, and a stash of 55mm filters, you'll still be able to use your old filters with the appropriate stepping ring.

When selecting your size, note that the first number given is the thread size for your lens and the second is the thread size for your filter. In the example above, the correct stepping ring size would be "52-55".