This short Psalm is a cry that God will faithfully respond to His own in the day of trouble. Such a day of trouble has apparently come and the appeal is threefold, firstly for YHWH to act in response to the past faithfulness of His people, secondly, because of YHWH's own trustworthiness as their covenant God, and thirdly and finally because He is their King.
It may be that we are to see it as in the form of prayer and response. First the leading intercessor makes his declaratory petition (Psalms 20:1-4). Then the people respond (5 a-b). Then again the leader speaks (5c), followed by further response (6-8; Psalms 20:6 may be the response of the high priest alone) and concluding with a final plea for deliverance (9).
Heading.
For the Chief Musician. A Psalm of David.
This is another Psalm offered to the organiser of the sacred music, or the choirmaster, and dedicated to David. The prayer is for God to aid the Davidic king, giving him victory against the enemies of God's people.