From the Super Mario Wiki, the Mario encyclopedia
A kadomatsu as seen on SNES Mario Circuit 1

Kadomatsu are traditional Japanese New Year decorations that are said to attract good luck. They consist of several tree cuttings, mainly bamboo stalks and pines, bound together with straw rope. They appeared in every course of New Year's Tour in Mario Kart Tour and are similar in function to the festive trees that appeared in prior tours. However, they were removed from the game at the end of the New Year's Tour.

Kadomatsu are placed around arches, entrances and finish lines and are usually colored white with a single Japanese character. Kadomatsu bounce and give bonus points when hit with an offensive item or when driven into while invincible or Mega, with the action being displayed as "Happy New Year!"