When Booth and Bones are leaving the cemetery Booth has Bones promise to make sure he is actually dead before burying him. Tradition used to be for a newly-buried person to have a string around their wrist leading to a bell that was above ground. Friends/family would sit at the graveside throughout the first night to listen for the bell to ring, signaling that the person had been buried alive and merely passed out instead of dead. This is where the term "graveyard shift" comes from, referring to a night shift at work.
With Booth being a recovering gambling addict, he should not have been participating in Hank's underground fantasy football league.