Celestial Starboard

The Celestial Starboard is an Expert mode-exclusive accessory obtained from the Moon Lord's Treasure Bag. It allows you to hover and boost upwards faster.

Trivia
It may be a reference to the Warp Star from the Kirby series

It could also be a reference to 'The More you know' meme because of the rainbow trail and tool-tip mentioning "The more you know"