Albert Barnes Commentary Psalms 97:1

"Jehovah reigneth; let the earth rejoice; Let the multitude of isles be glad." — Psalms 97:1 (ASV)

The Lord reigns - See the notes at (Psalms 93:1). This is the general fact to be considered; this is the foundation of joy and praise. The universe is not without a sovereign. It is not the abode of anarchy. It is not the product of chance. It is not subject to mere physical laws. It is not under the control of evil. It is under the government of a God: a wise, holy, intelligent, just, benevolent Being, who rules it well, and who presides over all its affairs. If there is anything for which we should rejoice, it is that there is One Mind, everlasting and most glorious, who presides over the universe and conducts all things according to His own wise and eternal plan.

Let the earth rejoice - The earth itself; all parts of it; all who dwell upon it. Since the earth everywhere derives whatever it has of fertility, beauty, grandeur, or stability from God; since order, beauty, and productiveness are diffused everywhere over it; and since it has received so many proofs of divine beneficence toward it, it has reason for universal joy.

Let the multitude of isles be glad thereof - The margin reads: “Many, or great isles.” The Hebrew is “many,” as are the Septuagint, the Latin Vulgate, the Chaldee, and the Syriac. The psalmist’s eye is evidently on the many islands scattered over the sea. Not merely the continents—the extended countries where nations dwell—have reason for joy. The beautiful islands also—those spots of earth that have risen from the deep and are covered with fruits and flowers—these too have reason to rejoice: to rejoice that God has raised them from the waters; that He keeps them from being overflowed or washed away; that He clothes them with beauty; that He makes them the abode of happy life; and that He places them in the expanse of the ocean, as He does the stars in the expanse of the sky, to beautify the universe.

The idea in the verse is that all the earth has cause to rejoice that Yahweh reigns.