The flame sensor exists to prove a flame across the burners. Without this flame sensor, there’s a risk of releasing gas without flame and risking an explosion upon ignition. The flame sensor is positioned on the opposite end of the burners from the igniter to ensure all burners ignite correctly.
Upon a call for heat, before the burners ignite, the furnace control board will send AC voltage to the flame sensor. This amount of voltage varies from manufacturer to manufacturer, but it’s usually between 80v – 100v. This voltage goes through a wire from the control board, to the wire that connects to the flame rod. You can think of it like the electrons are building up at the tip of the flame rod just waiting for a path so they can keep moving.