According to the simple interpretation of this Passuk, the Torah is teaching us not to profane our pledges by failing to execute them, rather we should fulfill our verbal commitments. The Chida interprets this Passuk in a novel way, sharing an important lesson about Tefila. He explains that one who doesn't profane their words by speaking indecently about things, ideas, or people, will have their words heeded to by Hashem. Since Hashem operates מידה כנגד מידה, measure for measure, if a person pays serious attention to the words that he utters, Hashem will pay close attention to what he has to say. In that fashion, "וכל היצא צפיו יעשה", Hashem will fulfill what emerges from this person's mouth - his Tefilos and Brachos will have tremendous power.