Horav Mendel, z.l., m’Kosov rendered this pasuk homiletically: Hashem will fulfill the blessing of the person who guards his mouth, who does not make his words chullin, mundane. In other words: if one sanctifies his speech, if he does not waste his words and everything that he says is of a holy nature, Hashem will make it His priority to fulfill this individual’s blessings.
It is told that Horav Yisrael Abuchatzeira, z.l., the Baba Sali, went to Ashdod in 1966 together with his father-in-law, Horav David Yehudiaf, z.l., to visit a venerable sage. The Baba Sali encouraged his father-in-law to ask the sage for a blessing, which he did. When they left, the Baba Sali told Rav David, “Did you notice that man’s hands, his skin, his eyes, his teeth? Judging by this person’s physical appearance, you would never believe that he is one hundred and thirteen years old! It happened that one hundred years ago, Horav Yaakov Abuchatzeira, z.l., asked this sage, who had then just become bar-mitzvah, to complete a minyan, which he did. The Rav then blessed him that he should be healthy and live to be one hundred and twenty years old. You see that the Rav’s blessing was fulfilled.” This is the meaning of “according to whatever will come from his mouth, He will do.”