This is a simple agreement used by a partnership to make a preemptive type of bid after finding a fit. The rule is straightforward -- If opener bids a suit (usuaully a major), and responder raises only to the two level, opener bids at the three level to close the bidding as a preemptive action. Opener MUST have more than 5 cards in the suit to do this -- otherwise, opener passes. If opener wishes to invite to game, opener bids another suit at the three level. Opener does not promise anything in the suit (some require it show at least 4 cards in the suit).
Hand 1 Hand 2
1H P 2H P
In Hand 1, opener must pass. In Hand 2, playing 1-2-3 stop, opener may bid 3H as a preemptive bid (thus forcing opps to enter the auction at the 3 level or higher -- an unlikely event).