It is a PCV vapor line that is part of the PCV system. The make up air comes from the tube into the engine. The crankcase vapors are drawn out via the PCV valve on the right hand valve covers. The vapors go into intake manifold where they are burned.
Get yourself some catch cans, two to be exact, 24 bucks each on ebay, I'm am gonna actually try to show some pics of mine, it has kept all of the blowby, besides the vapors out of my engine. Blowby is nasty stuff.
Or you can go to Ford and ask for a PCV plumbing, I wouldn't want that stuff in my engine, blowby that is.