Solomon's cage is a trap that is able to confine a demon indefinitely and is believed to be able to confine the spawn of the Devil. Sam was told that it could even confine the Devil himself, but the Devil laughed at the idea and said that it couldn't.
Shortly after Sam was thrown into the trap, his father was thrown in too. But after Tony had a change of heart, he pulled Sam out of the trap and saved him. But Sam's father was left behind, trapped in the cage and buried under a load of dirt.
Later, after all the rebel demons had left, Sam's mother opened the cage and dug her husband out, who appeared to be quite alive, indicating that he may not be human.