“set the city on fire.” Ai was one of the only three cities that God commanded that Israel burn after conquering it: Ai, Jericho, and Hazor (Josh. 6:24; 8:28; 11:11).
“You are to do according to the word of Yahweh.” Simple and straightforward. Humans are to obey God. This is one of the themes of Joshua: obeying God.