What does Joshua 8:12-26 mean?

He took about 5,000 men and set them in ambush between Bethel and Ai, to the west of the city. So they stationed the forces, the main encampment that was north of the city and its rear guard west of the city. But Joshua spent that night in the valley. And as soon as the king of Ai saw this, he and all his people, the men of the city, hurried and went out early to the appointed place toward the Arabah to meet Israel in battle. But he did not know that there was an ambush against him behind the city. And Joshua and all Israel pretended to be beaten before them and fled in the direction of the wilderness. So all the people who were in the city were called together to pursue them, and as they pursued Joshua they were drawn away from the city. Not a man was left in Ai or Bethel who did not go out after Israel. They left the city open and pursued Israel. Then the LORD said to Joshua, “Stretch out the javelin that is in your hand toward Ai, for I will give it into your hand.” And Joshua stretched out the javelin that was in his hand toward the city. And the men in the ambush rose quickly out of their place, and as soon as he had stretched out his hand, they ran and entered the city and captured it. And they hurried to set the city on fire. So when the men of Ai looked back, behold, the smoke of the city went up to heaven, and they had no power to flee this way or that, for the people who fled to the wilderness turned back against the pursuers. And when Joshua and all Israel saw that the ambush had captured the city, and that the smoke of the city went up, then they turned back and struck down the men of Ai. And the others came out from the city against them, so they were in the midst of Israel, some on this side, and some on that side. And Israel struck them down, until there was left none that survived or escaped. But the king of Ai they took alive, and brought him near to Joshua. When Israel had finished killing all the inhabitants of Ai in the open wilderness where they pursued them, and all of them to the very last had fallen by the edge of the sword, all Israel returned to Ai and struck it down with the edge of the sword. And all who fell that day, both men and women, were 12,000, all the people of Ai. But Joshua did not draw back his hand with which he stretched out the javelin until he had devoted all the inhabitants of Ai to destruction.