Psalms 99:2

The Lord is great in Zion - Compare Psalms 95:3. The meaning here is, not that God is “absolutely” great - which is indeed true - but that there is a sense in which he has shown himself great “in Zion;” that is, in his manifestations toward his own people. He has evinced power in their behalf; he has interposed for them in times of danger; he has so discomfited their enemies as to show that he is a great God - a God worthy to be adored.

And he is high above all the people - Above all the nations. He has them under his control. He rules over all. The God who rules in Zion also rules all the nations of the earth; and his people, therefore, have special occasion to praise him.