Definition of cap
What is meant by cap is that a given opponent's possible hand or range is cap at maximum strength. This deduction will be made according to the information obtained on the previous streets. A little confused? Nothing better than an example. You are on the button and open A2. The player on the small blind calls. You play your hand to the river and the board is A-A-K-Q-2. Your opponent passes and you bet. To your surprise, he pushes all in. Which hand does he have? Are you beaten? Answer: No, because his range is cap.
By cap, we mean that logically, our opponent does not have all his hands in his range. When you raised preflop, if he had AK or AQ, he would have rolled a 3bet, which he did not do. So when he pushes all in on the river, your chances of being dominated are minimal because your opponent's range is cap. He may have a hand like A5 overplayed or a hand like JT overplayed. He may also have a strange 22 for a small full hand. It's very important to realize when your opponent's rank is cap because it saves you from making disastrous folds and it encourages you to bluff well.