From the Super Mario Wiki
Green Rings are objects that appear in New Super Mario Bros. U. When the player jumps through one, Green Coins appear rotating in five groups of three, with one group appearing at a time and the next appearing after all coins in the previous group have been collected. They are similar to Red Rings, which generate Red Coins. When all the Green Coins have been collected, the player receives a power-up or a 1-Up Mushroom.