Joshua 3:3
And they commanded the people, saying, When ye see the ark of the covenant of the LORD your God, and the priests the Levites bearing it, then ye shall remove from your place, and go after it. And they commanded the people, saying, When ye see the ark of the covenant of the LORD your God, and the priests the Levites bearing it, then ye shall remove from your place, and go after it.
The command required Israel to watch for the priests carrying the ark of the covenant and, when they moved, to break up from their place and go after it. That movement began at the center of the camp, where the ark rested, and proceeded toward the Jordan, and the people were to follow that advance. The ark marked the Lord's presence going before Israel to lead the way through unknown ground. To follow the ark was therefore to follow the Lord's own movement, keeping His leading in sight and conforming every step to it.