All bridge played follows the rules of Contract Bridge, sometimes just called bridge. Once bridge became a popular game, players have been trying to come up with ways to improve their bidding and playing the hand. These are called conventions. Some have work, and some have not. Astro is a response convention for use with an opening bid of 1NT
The response to the opening bid of 1NT provides a description of the make-up of the responding partner. The convention is used with the responder has 10+ points in his hand.
2 Club Shows Hearts and a minor suit with a 5-4 or 4-5 split
2 Diamonds Shows Spades and another suit with a 5-4 or 4-5 split
2 Hearts Indicates 5 Hearts
2 Spades Indicates 5 Spades
2NT Shows clubs and diamonds (usually a 5-5 split). Opener should respond with his preference by replying with 3 Clubs or 3 Diamonds
With this information the opener can make his overcall and set the bid suit.