Step up rings are used to fit a larger filter on to a smaller lens filter thread. They are a useful way to reduce the number of different filter sizes you need to get. They are inexpensive to buy and they are available in just about any combination of lens filter thread and filter size.
In principle you could also use step up rings to fit square filter holders on to different lenses, but in practice these use bespoke lens filter thread adaptors and you would not really want to use both a holder adapter AND a step up ring.