It takes raw Iron ore on the left, and deposits it on the conveyer to the right. You can see the filters on the two rightmost inserters, and the logic for the leftmost ones are as follows:
Top > 4 Iron Plates in the crate.
Bottom < 3 Iron Plates in the crate.
Basically the logic stops more ore from being inserted into the furnace once the required number for a Steel Plate have been forged, then once the first plate from the box enters the furnace it switches the furnace to Steel production until the Steel Plate is finished, at which point it becomes available for Iron production again.
