Attempting to steal boards is a common winning strategy at every level of bridge. Your opponents hope to buy the auction on a board that belongs to you. If the opponents make their contract or go down less than you would have scored, they have successfully stolen the board from you.
Penalty doubles are your best defense against opponents attempting to steal boards. The difficulty is that it isn't always clear whether you should double for penalty or take another bid. Sometimes you want to consult your partner on the decision. You don't want to pass, expecting partner to bid or double, and have partner pass, allowing the opponents to play undoubled. Collecting 50, 100, or 150 for beating their contract is poor compensation for your making game contract.
To safely consult partner on a high-level decision, you need to agree on when the opponents cannot buy the contract undoubled. In these situations your pass is forcing. If you aren't sure whether to double or bid, you can pass. Since you have agreed the opponents cannot play the contract undoubled, your pass is forcing (i.e. a Forcing Pass). Your partner must bid or double.
For example, when you are in a game forcing auction and your opponents try to buy the contact, you cannnot allow them to play undoubled. This is a Forcing Pass situation.
In the above auction, North's 2NT response is an artificial, game forcing heart raise (Jacoby 2NT). East attempts to steal the board by bidding 4♠. East may believe 4♠ has a chance of making or may be sacrificing. North-South must decide whether to double 4♠ or bid on to 5♥. The one thing North-South shouldn't do is defend 4♠ undoubled. If South wants to penalize 4♠, South can double. If South wants to compete to 5♥, South can bid 5♥. If South wants to make a slam try the can make a Control Bid or ask for key cards. Sometimes South will not know how to proceed, and will want to consult their partner. With the understanding the passes are forces when the opponents compete in your game forcing auctions, South can pass to give North the opportunity to use their judgment on whether to double or bid more. Because South's pass is forcing, North cannot allow pass and allow East to play 4♠ undoubled.
We highly recommend reading Steve Robinson's "What's a Forcing Pass" article. Sreve provides his own advice and asks his expert panel to share their agreements on when a pass is forcing.