What does Deuteronomy 31:3 mean?

The Lord your God is the one who will go ahead of you. He'll destroy those nations in front of you, and you'll take their land. Joshua will lead you, just like God said. Deuteronomy 31:3 - Modern Text Bible

(Moses reassures them that God is still in charge, and Joshua is the new leader. They're not alone in what's coming.)