As the Israelites entered Canaan, they prepared to fight the worshipers of the gods of the land of Canaan. They feared these gods might help the people who lived in Canaan to win. To encourage Joshua, God used his personal name, the LORD (Hebrew Yahweh; see study note on Exodus 3:15). This was to remind him that the LORD was more powerful than any false gods the Canaanites might ask to help them.
your God is with you: God's constant presence enabled Joshua to follow God's instructions.