Doubling is a way of raising the stakes during a game. The basics are simple. At the beginning of a game you are playing for a single stake (a point in match play or a money stake). The doubling cube starts at one (although most cubes do not have a 1 on them, so they are set on the 64 or some other large number) and centered between the players, usually at the side of the board.
If a player thinks they are ahead in the game and likely to win they may offer their opponent a double. A player may only double on their turn and before they roll the dice. The cube is placed on the board with the number 2 facing up. The player doubling then says “double.” At that point, their opponent has the option of “passing” or “dropping” the double, ending the game and paying the current single stake (or conceding the point in match play) or “taking” the double, continuing the game but at twice the original stake.
A player who takes a double then literally takes the cube and places it on their side of the board with the 2 showing. That player now “owns” the cube. If the situation changes later in the game, and the cube owner now thinks they will win, they can offer a redouble, raising the stakes to 4. The same procedure applies: the doubled player may drop and pay their opponent twice the original stake, or take the cube, doubling the stake again to 4 times its original value. The new cube owner now has the option to double again later in the game, raising the stake to 8 times its original value (if taken), and so on.
Those are just the mechanics of using the cube, of course; the real skill comes in knowing when to double, when to take, and when to drop. That’s where things get complicated, and interesting. Check out the links below to a few articles that will give you more information on the basics of doubling strategy.