He's actually saying "FEEEEED MEEEEE!" Barn owls nest in areas that are also accessible to a variety of predators. They probably evolved this particular "feed me" noise to frighten away cats, weasels, raccoons & such or at least keep them at bay until mommy or daddy gets back home.
I love owls, and barn owls are my favorite. The owl at the raptor center where I volunteer makes a cute noise like this sometimes. He also snaps his beak and then sticks his tongue out. It's adorable.