This is a song of purest praise, probably sung at times when the armies of Israel were victorious over enemies. Such enemies all believed in many gods - gods of floodwaters, gods of earthquakes and fierce winds, gods of wild animals, etc. In this song, though, all the things supposedly done by these other gods are ascribed to the one true God - Jehovah - Who is over ALL things.
The imagery is powerful, particularly for those who have observed such natural disasters as raging floods, earthquakes, tornadoes, hurricanes, etc. Those who sing (the "mighty ones" = warriors of the army) are exhorted to praise Jehovah - the One Who is truly over ALL things in nature. They should praise Him "in the beauty of holiness." Because they should know Jehovah as the only true God, they are therefore not like the world that believes in many gods - they are "holy." "Holiness" just means separateness, uniqueness, and it's the privilege of Israel - uniquely - to praise the true God as His separated people.
So - Who controls the mighty forces of the floodwaters or the roaring torrents that seem uncontrollable? God's people have the answer. They sing, "The voice of the Lord is over the waters; the God of glory thunders; the Lord is over MANY waters. The voice of the Lord is powerful; the voice of the Lord is full of MAJESTY!"
Then the next few verses, full of the powerful imagery about earthquakes, splintering of great trees, dividing of otherwise indivisible flames of fire, stripping bare of forests, and forcing the sudden birth of deer calves - are calculated to move the worshipers to a sense of awe and reverence before the power of God!
Having that awe in their hearts, they're ready to sing about the true Lord of all things. They sing, "The Lord sat enthroned at the flood (the great flood of Noah's day), and the Lord sits as King forever! The Lord will give strength to His people; the Lord will bless His people with peace!" The great GOD Whose awesomely powerful works they've just considered, the GOD Who has been Lord from the beginning and will be Lord to the end - is the God Who can also give them peace in the midst of all their enemies!
How about you? Are you adequately awed at the immense and glorious works of God that are around you every day - the sun, the moon, the stars, the mountains, etc., etc.? These are the works of the Agent of creation - your SAVIOR! Jesus Christ is Lord of all! His works therefore are meant to move you to praise and thanksgiving, because, though they reveal such a majestic Lord of all, yet He also cares for YOU!