The doctrine, in pre-emptive bidding, that one should not risk going down more than two tricks, if vulnerable, and three tricks if not vulnerable.
If you adhere to the rule, the opponents double you and partner has a useless hand, you will lose a penalty of 500, broadly the same as an opposing game.
With the rule, if you have seven tricks in your hand, you can pre-empt to the four level non-vulnerable or the three level vulnerable.
Many modern players pre-empt a little more aggressively than the rule of two and three, partly because the opponents may have a slam if partner has a bust and partly because the opponents rarely double for penalties.