In the Old Testament, the relationship between the Lord and Israel’s king can be compared to a suzerain-vassal relationship. A suzerain was a great king. A vassal was a smaller king who served him.
In this picture, the Lord is the great King. Israel’s king rules under his authority (see Deuteronomy Book Introduction, “Literary Form”). The Lord invites his anointed king to ask for what he needs (see Psalm 21:2, 4).
All nations will be subject to David's descendant, the Messiah.