The air is drawn in from the room at the back through a filter (to remove environmental particles) and then pushed out. The laminar flow means that any contaminating particles introduced into the hood (i.e on the surface of glassware, on your labcoat) are pushed in a straight line out of the hood. You work from left to right (or right to left) to minimise contamination.

So, for instance, if I was pouring agar plates I would start from the top right hand corner and set out a line of plates, pour those plates, move forward, set out a new line and then pour those plates. You never cross over the "sterile" environment.