What does Psalm 21:2 mean?

In Psalm 20:4 the congregation prayed for the Lord to grant David his heart's desire. His heart's desire was victory over his enemy in an upcoming battle. Now, the congregation thanks the Lord for giving David his heart's desire. He had gained the victory over the enemy.

When the heart's desire aligns with the Lord's will, the Lord answers prayer and causes the heart to rejoice. We know from 2 Samuel chapter 7 that the Lord had plans for David. He promised to give David rest from all his enemies (2 Samuel 7:11) and to give him an everlasting kingdom (2 Samuel 7:16). If we pray for things that cater to sinful desires, the Lord will not hear us, but if we trust in the Lord and delight in Him, He will give us the desires of our heart (Psalm 37:3–4). Jesus instructed us to ask of the Father in His name (John 16:23). He promised, "Ask, and you will receive, that your joy may be full" (John 16:24).

The term transliterated into English as "selah" is extraordinarily difficult to translate. Scholars think it might be related to terms implying a rest, or to praise, or even to a musical interlude. The word appears often in the Psalms, but is also used three times by the prophet Habakkuk (Habakkuk 3:3, 9, 13). Most likely, it's something inserted in the same style as modern writers might include an "amen!" or "hallelujah!" in between lines of a song or poem.