Coin ring
A Coin ring is a collectible found in most/all kingdoms. If Mario or Cappy passes through this ring, it will disappear and the coin counter will go up by three. While these rings are not required to complete the game, they speed up the process of collecting 9999 coins. These rings first appeared in Super Mario 3D Land, with the only difference being that they gave five coins instead of three.
A red variation of these rings exist (they primarily appear in 2D games), yet they do not appear in this game.