Some say that need teaches one to pray, but that is not always the case. Need can also cause one to curse and say bad things. That is the fear of the poet. He prays, in two parallel requests, that God will guard him from speaking evil words, that he will set a watch before the opening of his mouth. Let only good and kind words pass the gate of his lips.
3 Set a guard, O LORD, over my mouth; keep watch over the door of my lips!