Psalm 21 is a depiction of a victorious king - undoubtedly a prophetic reference to the One true King - Jesus. If the prior psalm - psalm 20 is a prayer that speaks of the king's blessing upon his army, psalm 21 is a bestowal of blessings upon the king himself.
The author of this psalm is David, the greatest king of Israel. In it, he acknowledges that it's God's strength and power that gave him the victories he accomplished as a king. Let's listen in to him, "The king rejoices in your strength, LORD. How great is his joy in the victories you give!"
Kind David recognizes God as an answering and giving God who grants his heart's desire and has not withheld his requests. God also gave him unending blessings foremost of which is eternal life and joy in His presence. And because he trusts in the Lord, he will not easily falter. These are wonderful royal benefits that king David experienced yet they are also available to anyone who is a child of God. Just come to think of it, as sons and daughters of the King and Ruler of the universe, we have all these royal benefits in Christ for we belong to His kingdom! (read Ephesians 1).
From verse 8 till the end of this psalm, king David turns his attention to the power, strength, and might of the one true King. This King will one day appear for battle and will burn his enemies up as in a "blazing furnace...and His fire will consume them." Talk of divine retribution - while we are looking for true justice in the world today against evil atrocities it will not come until the coming of this one true King. Human justice is usually corrupt, unfair, and does not go far enough. But divine justice is perfect, fair, and swift.
And so with David we shout, "Be exalted in your strength, LORD; we will sing and praise your might."