Cubelets are a crate-like system, a new way to reward you for playing. You can get them by playing games - completing achievements and challenges and doing well in the games - or by voting .
You can open them at the "cubelet opening machines" - End Portal Frames - to win a random Vanity item. Before revealing your prize a firework trail will summon above the block.
Qualities of the item range from Common to Mythical (Common, Uncommon, Rare, Mythical, Legendary). Some items will have other requirements - for instance some item must be found from Cubelet and the founder must have premium account.