The most quoted Old Testament chapter in the New: David's Lord seated at God's right hand, a priest forever in Melchizedek's order. Jesus asked the question; Hebrews gives the answer.
The Shadow
One verse, two Lords. The Lord said to my Lord: sit at My right hand. David, the highest king Israel ever had, calls someone his own Lord, and that someone is invited to sit, enthroned beside God, while his enemies are made a footstool. Then verse 4 does something the law of Moses flatly forbade. It makes this seated Lord a priest. Not from Levi's line, where every legitimate priest had to come from, but forever, according to the order of Melchizedek — the king-priest who met Abraham before Levi was born. Israel's kings could not touch the priesthood; Uzziah tried and left the temple a leper. The psalm holds open an office no one in the nation could legally fill: a King who is also a Priest, seated, waiting.
The Fulfillment
The New Testament quotes this chapter more than any other in the Old, and Jesus started it. In His final week He put verse 1 to the Pharisees: if the Christ is David's Son, how does David call Him Lord? No one could answer. At Pentecost Peter supplied it: David did not ascend into the heavens — Jesus did. Hebrews then builds three chapters on verse 4: a priest forever, from the wrong tribe on purpose, because the oath came later than the law and outranks it.
And Hebrews 10 merges the two halves of the psalm into one picture: after He had offered one sacrifice for sins forever, He sat down, from that time waiting till His enemies are made His footstool. The posture is the gospel. Standing priests never finished. The seated One did.
Him All Along
Your High Priest is not on His feet working; He is on a throne waiting. The sacrifice is behind Him, and only the mop-up remains ahead. Your standing with God does not wait for the last enemy to fall — the session has already begun, and you are represented there by a Priest whose term never expires.