This will be a one-photo post so you can zoom in. These little circular things allow air to move across the underside of the roof, to prevent condensation. This is a somewhat more complex method than what is usually employed in home building. Earlier posts showed gray styrofoam channels attached to the underside of the roof, but above the insulation. These vents are the openings that allow the air to flow through those channels, and keep the roof dry, and mold-free. This is the same thing as the vented soffits on most (perhaps your) houses. Look underneath your roof overhang, and you'll notice that the panels underneath have small openings. Those are a different version of what you see depicted here. We can all breathe easier now.