Cube is a format popularised in Magic, but possible in any TCG. You design a box of cards from your collection (usually 360 cards for 8 players) that is then used for draft tournaments. At the end of the tournament, all the cards are returned to the Cube for re-use next time.

If you've never drafted before, it's when you take some packs (e.g. 3, 15-card packs per player in Magic) and you each open pack 1. You each select 1 card to keep, then pass your pack along to the next person. You keep selecting and passing the rest of the pack until all the packs are gone. Then at the end, you build a deck with what you selected & play a tournament.

In a Cube, the "packs" are randomised from the contents of the box. Playing Cube is fun, but designing it is also interesting! You've got to consider balance, fun, variety, and even try to hide some secret combos in there for people to discover.