Waiting patiently for the Lord is an experience of faith we must grasp. The first half of this verse describes the state of our faith; the second half the assurance we hold, in our innermost being, that God has looked upon us with compassion.
David frequently spoke of waiting for the Lord. He knew how to call upon God, but also how to wait for Him. If David only knew how to call upon God but not how to wait for Him, his faith would have failed him over and over.
David's own son, Absalom, seized his throne by force. At the same time, his counselor, Ahithophel, betrayed him (2 Sam 16:15-23). In Psalm 62:4, David wrote of how they fully intended to usurp him and cast him down from his position. Yet he prayed, "My soul, wait silently for God alone, for my expectation is from Him" (Ps 62:5). Before long, his hope was realized: he was restored to the throne.
In addition to his son's rebellion and the betrayal of his officials, David was afflicted with sores and boils all over his body. "Have mercy on me, O Lord, for I am weak; O Lord, heal me, for my bones are troubled," he prayed in Psalm 6. Enduring this great pain, he wrote: "My soul also is greatly troubled; but You, O Lord—how long?... All night I make my bed swim; I drench my couch with my tears" (Ps 6:2-3,6). Yet he concluded his psalm with the conviction that the "Lord has heard my supplication; the Lord will receive my prayer" (Ps 6:9). Later, God indeed listened to David's supplication and healed him.
Knowing how to wait for the Lord reveals David's sincere trust in God, but it is also the secret to the many blessings he received. Only with a faith that waits patiently for the Lord can you experience the grace that "the Lord has heard [your] supplication." If we follow David's example by waiting patiently for the Lord in whatever we ask, we will continuously experience the grace that David experienced.