What exactly is Legendary Cube/Vintage Cube in MTG Online?
In general 'Cube' is a Magic format where a person creates their own custom draft environment, choosing which cards are included. They can take cards from any sets the creators choose and can have any sort of theme their creators choose.

The Legendary Cube is one such cube created by Wizards of the Coast that is meant to showcase Legendary Creatures. Originally every creature was legendary, though it has since been updated to include non-legendary ones too. Also it was used to release some cards from various Commander products that hadn't been released in digital form yet.

The Vintage Cube (previously known as the Holiday Cube due to only being available around December) is another cube developed by Wizards of the Coast that is designed to be a 'power cube' or a cube that has many of the most powerful cards ever released in the game. This cube doesn't really have a rotation like Standard, but they do change the cards in it slightly in order to add cards from new sets and vary the environment slightly.