You use the air vents in the Refreshing mode. In this mode you open the air vents by putting the air vent slides in refreshing position. When you press the refreshing button, air starts to come out of the ironing board and will pass through the fabric of your garment, removing unpleasant smells.
When you iron and steam, the air vents have to be closed. Close the vents by setting the two air vent slides to the ironing position.