Definition of stab
When we say that a player stab to an pot, it means that he is betting to steal the pot. A "stab" in English is a dagger shot. The bet is a shot at the pot that is thrown to win it without being strong; it is an opportunistic move. Usually a stab to pot is called a stab when no one seems interested in the pot. Imagine a family pot. Action on flop makes check-check-check-check. Then comes the turn and the action is check-check-check, and this time the last player to speak says that since no one seems interested in pot, it might be time to call stab to flop, to steal it.
What is meant by "stab"?
As mentioned above, a stab does not indicate an incredible hand. A stab does not indicate a square of kings. One stab to steal the pot, because the players look disinterested. And usually the stab will come on the turn or the river. The player who stab had a lot of information to judge that nobody was interested in pot and that it was the right time to make a move.