This is an "above ground" hole reducer for putting practice. The idea is that this goes inside the cup at your local practice green to reduce the effect size of the hole, by limiting the size of the opening where the ball can enter the hole.

One of the advantages of using a hole reducer like this, rather than the "ring" type reducer, is that it's easy to remove the balls out of the hole.

Offered here are 4 different sizes of openings.