Definition of stab
When a player is said to stab at a pot, it means that he is betting to steal the pot. A "stab" in English is a stab at the pot. A stab is a move to win the pot without being strong; it is an opportunistic move. Generally, a stab at the pot is when no one seems interested in the pot. Imagine a family pot. The action on the flop is check-check-check. Then comes the turn and the action goes check-check-check, and this time the last player to speak thinks that since no one seems interested in the pot, it might be time to stab on the flop, to steal it.
What is meant by "stab"?
As mentioned above, a stab does not indicate an incredible hand. You don't stab with four kings. You stab to steal the pot, because the players look uninterested. And usually the stab will come on the turn or river. The stabbing player has had plenty of information to judge that no one is interested in the pot and that it is a good time to try.