What did the priests carry into the Jordan ahead of the people?

Answer: The ark of the covenant

The people followed the ark of the covenant carried by the Levitical priests; when the priests' feet touched the water, the Jordan stopped.

“and commanded the people: “When you see the ark of the covenant of the LORD your God being carried by the Levitical priests, you are to set out from your positions and follow it.”— Joshua 3:3 (BSB)