“The LORD said unto my Lord, Sit thou at My right hand, until I make Thine enemies Thy footstool.” Psalm 110:1 (KJV)

What a scene the Psalmist is seeing in heaven between the Father and the Son.  David has a great interest in this Second Person; he calls, “my Lord”.  As should all who have an interest in spiritual matters.

Of this verse C. H. Spurgeon can say it much better than myself.  Here are his words on this verse;

“The Lord said unto my Lord” – Jehovah said unto my Adonai: David in spirit heard the solemn voice of Jehovah speaking to the Messiah from of old. What wonderful intercourse there has been between the Father and the Son! From this secret and intimate communion spring the covenant of grace and all its marvellous arrangements. All the great acts of grace are brought into actual being by the word of God; had he not spoken, there had been no manifestation of Deity to us; but in the beginning was the Word, and from of old there was mysterious fellowship between the Father and his Son Jesus Christ concerning his people and the great contest on their behalf between himself and the powers of evil. How condescending on Jehovah’s part to permit a mortal ear to hear, and a human pen to record his secret converse with his co-equal Son! How greatly should we prize the revelation of his private and solemn discourse with the Son, herein made public for the refreshing of his people! “Lord, what is man that thou shouldst thus impart thy secrets unto him.”
Though David was a firm believer in the Unity of the Godhead, he yet spiritually discerns the two persons, distinguishes between them, and perceives that in the second he has a peculiar interest, for he calls him “my Lord.” This was an anticipation of the exclamation of Thomas, “My Lord and my God,” and it expresses the Psalmist’s reverence, his obedience, his believing appropriation, and his joy in Christ. It is well to have clear views of the mutual relations of the persons of the blessed Trinity; indeed, the knowledge of these truths is essential for our comfort and growth in grace. There is a manifest distinction in the divine persons, since one speaks to another; yet the Godhead is one.
“Sit thou at my right hand, until I make thine enemies they footstool.” Away from the shame and suffering of his earthly life, Jehovah calls the Adonai, our Lord, to the repose and honours of his celestial seat. His work is done, and he may sit; it is well done, and he may sit at his right hand; it will have grand results, and he may therefore quietly wait to see the complete victory which is certain to follow. The glorious Jehovah thus addresses the Christ as our Saviour; for, says David, he said “unto my Lord.” Jesus is placed in the seat of power, dominion, and dignity, and is to sit there by divine appointment while Jehovah fights for him, and lays every rebel beneath his feet. He sits there by the Father’s ordinance and call, and will sit there despite all the raging of his adversaries, till they are all brought to utter shame by his putting his foot upon their necks. In this sitting he is our representative. The mediatorial kingdom will last until the last enemy shall be destroyed, and then, according to the inspired word, “cometh the end, when he shall have delivered up the kingdom to God even the Father.” The work of subduing the nations is now in the hand of the great God, who by his Providence will accomplish it to the glory of his Son; his word is pledged to it, and the session of his Son at his right hand is the guarantee thereof; therefore let us never fear as to the future. While we see our Lord and representative sitting in quiet expectancy, we, too, may sit in the attitude of peaceful assurance, and with confidence await the grand outcome of all events. As surely as Jehovah liveth Jesus must reign, yea, even now he is reigning, though all his enemies are not yet subdued. During the present interval, through which we wait for his glorious appearing and visible millennial kingdom, he is in the place of power, and his dominion is in no jeopardy, or otherwise he would not remain quiescent. He sits because all is safe, and he sits at Jehovah’s right hand because omnipotence waits to accomplish his will. Therefore there is no cause for alarm whatever may happen in this lower world; the sight of Jesus enthroned in divine glory is the sure guarantee that all things are moving onward towards ultimate victory. Those rebels who now stand high in power shall soon be in the place of contempt, they shall be his footstool. He shall with ease rule them, he shall sit and put his foot on them; not rising to tread them down as when a man puts forth force to subdue powerful foes, but retaining the attitude of rest, and still ruling them as abject vassals who have no longer spirit to rebel, but have become thoroughly tamed and subdued.

For further study consider Matthew 22:42-44;  Acts 2:33;  Hebrews 7:4;  2 Peter 1:17.