יִמְלֹ֤ךְ יְהוָ֨ה ׀ לְעוֹלָ֗ם אֱלֹהַ֣יִךְ צִ֭יּוֹן לְדֹ֥ר וָדֹ֗ר הַֽלְלוּ־יָֽהּ׃
시온아 여호와 네 하나님은 영원히 대대에 통치하시리로다 할렐루야
Rashi
The Lord will reign forever He will perpetuate His kingdom with the redemption of His children.
Radak
ימלוך. "(He) Reigns": And when will this be? When the Lord reigns forever, your God, O Zion, from generation to generation, as it is said, "And the Lord shall be king over all the earth," (Zechariah 14:9) meaning the entire world will recognize His kingship. Zion is mentioned because from there, instruction will go forth to the whole world, and they will acknowledge Him as king over everything after He executes judgment on the wicked, as in the Valley of Jehoshaphat. Or when the Lord reigns, all the people of the world will praise the Lord (Haleluya) and recognize His kingship, and then your song will be good and pleasant, and your praise will be perfect.
Ibn Ezra
"May he reign" - If God is so, he is fitting to be king and only for Him is the indulgence of kingship, and the poet concludes as he began [with הללויה].