I think you are referring to being good 2 out of 3 times.

You are laying 2-1 on the river bet.

If it is checked to you for instance, and you are heads up and last to act, you can bet or check. If you bet you are trying to win 1 bet. You risk getting raised in doing so, and losing 2 bets when you could have checked for no risk. So in this simple example, you would have to be good over 2 out of 3 times to show a profit.

Please note, the above example assumes that you get raised every single time you are beat, which is not the case. Other examples I have read about your hand being good use a % of 55.