What is a step up and step down ring?

A step-up ring enables you to fit a filter with threads larger than your lens.

On the contrary, a step-down ring does the opposite.

If you have 72mm lens threads and want to fit a 77mm filter, you need a step-up ring. If you have 77mm lens threads and want to fit a 72mm filter, you need a step-down ring.

Step-down rings are not as often used as step-up rings because, in most cases, the smaller filter sizes may result in vignetting.