Now the priests bearing the ark of the covenant of the LORD stood firmly on dry ground in the midst of the Jordan, and all Israel was passing over on dry ground until all the nation finished passing over the Jordan.
Moving into the book of Joshua, and Joshua has taken over from Moses. He is now leading Israel across the Jordan river into the promised land to begin the conquest of the nations there.
Crossing over the river on dry ground is a sign similar to that of Moses parting the sea. This shows that God is with Joshua as he was with Moses, helping the people to trust Joshua as they trusted (or should have trusted) Moses.
The priests bearing the ark were standing in the midst of the river while the people passed across. The ark is a sign of God's presence during the miracle, so that people would not be able to attribute the amazing events to another god or person. And after the ark left the dry river bed, the water began to flow again.
The conquering of the nations begins with the city of Jericho.