Joshua is the new leader of Israel and is instructed by God to lead Israel into the promised land. But they first have to cross the Jordan river. Here, God will show His power by stopping the Jordan river so that the Israelites can walk across and into the promised land (just like how God did this with Moses in parting the Red sea so that the Israelites can walk across and escape from the Egyptians). God was going to do this by having the priests carry the Ark of covenant, which shows God’s presence with Israel and His holiness, into the middle of the Jordan river and stay there until everyone crossed. Finally, God doesn’t want them to forget about this event. So, He has Joshua choose 12 men, one frpm each of the 12 tribes or families of Israel to pick a stone from the Jordan river. This stone is to be kept and remembered as a sign of what God has done for them at the Jordan river so that they and their children and children’s children and so on will know and remember this.