What does Exodus 15:18 mean?

This statement is an exclamation to end a song of celebration (Exodus 15:1). God has just rescued Israel in spectacular fashion (Exodus 14:21–28). Over the course of this song, the people have recounted those events (Exodus 15:2–12) and looked ahead to entering the Promised Land (Genesis 17:8; Exodus 15:13–17). Now, they proclaim the Lord's eternal rule.

Scripture emphasizes the duration of the Lord's reign. There will be no limit to how long the Lord leads His people. This is true both in this world and for eternity. The Lord has a plan to ultimately recreate a new heaven, a new earth, and even a New Jerusalem (Revelation 21:1–3) where all His people, Jewish and Gentile (Revelation 7:9), will worship Him for all eternity. This is the climax of God's redemptive story. Revelation 22:5 uses the same words, noting the Lord will "reign forever and ever."