"Hashem is my shepherd; I shall not want"
Here, Rabbi Shimon tells us that the Shechinah came and dwelled on David and inspired him to recite this praise to God and to request sustenance from Him. When God brings food to the world, the Shechinah takes from it first, so it is for her sake that food descends to all the worlds. "the Creator is my shepherd," we learn, also means that He sustains a person with everything he needs.
The Light of sustenance and abundance shines upon us through the merit of King David. We learn from David that one should meditate upon this passage with great desire, asking the Creator to fill all of our needs, for if one does not ask, one cannot receive. Thus, ask for everything good -- the totality of Light, the Final Redemption, the annulment of judgments, and infinite loving kindness.