What's an MTG "cube"?

The shortest definition of a "cube" for Magic: The Gathering is that it uses existing cards to form a custom set that's built for drafting. A lot of the particulars can vary, but those core elements are the same pretty much all the time. Each of those core elements is broken down below.

Magic: The Gathering. First of all, a cube is a different way to play Magic. If you don't know the basics of Magic, getting started with a cube would be a rough introduction! If you know how to play standard 2-player Magic and understand typical cards, you're totally ready to try out drafting (explained more here), and hey, why not with a custom set?

Existing cards. A cube isn't "homemade" in the sense of including totally new cards. Just the opposite! All the cards in a cube have been released before, in real expansion sets. No homebrewed cards here.

Custom set. Over 20,000 unique cards have been printed in Magic's lifetime — what sets one cube apart from another is which of those cards are included. The maker of a cube can choose whatever restrictions they like, or no restrictions at all. Many cubes consist of only the most powerful cards ever released. Other cubes focus on cards that create a lot of randomness or chaos. Some only draw from a single expansion or block, essentially recreating a draft format from the past. Some evoke a totally new setting through card choices. And some limit themselves to cards universally considered total junk. Picking the theme is often the first step in making a cube.

Built for drafting. The minimum number of cards needed for a typical Magic draft is 360. A normal draft has 8 players, each with 3 packs of cards, and each of those packs containing 15 playable cards (in other words, 8 x 3 x 15 = 360). That's just the minimum number, and many cubes include more. With only the bare minimum, every single draft will have the exact same card pool at the table, and surprises are part of the fun! So if a cube had 720 cards, since only 360 are used for a draft, only half the possible cards will be used in any one draft. Next time, when you shuffle up the set, the pool will change again. And then again. And again...