"Remain" probably represents sitting on the throne as king. Alternate translation: "Yahweh sits on his throne forever" or "Yahweh rules forever" (See: [[rc://en/ta/man/jit/figs-metonymy]])
The phrase "his throne" represents God's rule. Possible meanings are 1) "He rules in order to judge people" or "He rules over people justly" (See: [[rc://en/ta/man/jit/figs-metonymy]])