A robe is an accessory that can be worn by the player. You can obtain it by winning a giveaway in #all-hail-pixelbot that has the robe as a reward.
Like capes, the robe is not an in-game item. Once you obtain it, linked your accounts (More info in PixelLink) and an Admin has messaged you, you can activate it in-game using
/redeem (More info in Commands). You can toggle the visibility of the robe by using
/redeem toggle robe