Thy seed will I establish for ever - Thy children; thy posterity. The reference is to his successors on the throne. The promise was that there should not fail to be one on his throne; that is, that his dynasty should never become extinct. See 2-Samuel 7:16 : "And thy house and thy kingdom shall be established forever before thee: thy throne shall be established forever." Compare also 1-Kings 2:4. The word rendered "establish" means properly to fit; then, to make firm; to put on a solid basis.
And build up thy throne - It shall be kept up; it shall be like a building that is constantly progressing toward completion. The meaning is, that it would not fail. He would not begin the work, and then abandon it. The dynasty, the kingdom, the throne, would be complete and perpetual.
To all generations - As long as the world should stand. This can have been accomplished only by the Messiah occupying in a spiritual sense the throne of "his father David." Compare Luke 1:32-33.

Thy seed will I establish for ever, and build up thy throne to all generations - And this covenant had most incontestably Jesus Christ in view. This is the seed, or posterity, that should sit on the throne, and reign for ever and ever. David and his family are long since become extinct; none of his race has sat on the Jewish throne for more than two thousand years: but the Christ has reigned invariably since that time, and will reign till all his enemies are put under his feet; and to this the psalmist says Selah. It will be so, it is so; and it cannot be otherwise; for the Lord hath sworn that he shall have an euerlasting kingdom, as he has an everlasting priesthood.

Thy seed will I establish for ever,.... Meaning not the natural seed of David, at least not only them; whose family was indeed preserved, though in very low circumstances, until the Messiah came, who sprung from thence, Luke 1:27, but the spiritual seed of Christ, to whom it was promised that he should have a seed, and should see and enjoy it, and which should endure for ever; see Psalm 89:29, and so he always has had a seed to serve him in all generations, in the worst of times, and will; and who are established in him, and will be kept and preserved by him, and whom he will present to his Father, saying, "Lo, I and the children whom thou hast given me", Hebrews 2:13.
and build up thy throne to all generations; and this shows that the passage is not to be understood literally of David, and of his temporal throne and kingdom, which did not last many generations; but of the spiritual throne and kingdom of the Messiah, who sprung from him, called the throne of his father David, whose throne is for ever and ever, and whose kingdom is an everlasting kingdom, Luke 1:32, Psalm 45:6, his throne is in the heavens, where he will reign until all enemies are put under his feet; and it is also in the midst of his church, and in the hearts of his people, where he reigns as King of saints; and he is on the same throne with his Father; it is the same with his, as to glory, power, and authority; on this he will sit, and judge the world at the last day; and on it he will reign with his people a thousand years, in the New Jerusalem state, and after that to all eternity, Revelation 3:21.
Selah. See Gill on Psalm 3:2.

Build up - I will perpetuate the kingdom to thy posterity; which was promised upon condition, and was literally accomplished in Christ.