One prominent theme in Joshua is "obedience." In chapters 3-4, the people are mobilized to head out to conquer the promised land. They were prepared to do so, they consecrated themselves, and then they headed out. But all this was done in the context of obedience, as they obeyed God's commands.
As they crossed the Jordan River, and the water stopped flowing as they obeyed God's commands, 12 men, one from each tribe, picked up a stone to build a memorial that would stand as a reminder of what God did, for themselves, and for future generations.
How do we mobilize and prepare ourselves for God's service? How do we remember him? How do we insure that our future generations will be taught him?