Guy doesn't know what it means, he doesn't even know what he'd do if he got it, if you were to ask him what it is he'd reply:
"It's like... right there... y'know? It's riiiiiiight there."
It was a line. A line where the land and sky met.
One thing he did know however... was that he'd stop at nothing to get it.
The bet was simple: "Find a demon, bring back its head, and you get to go on a real demon job with me and the others. But if you come back empty handed, you quit smoking."