The Question and Answer section for Merchant of Venice is a great resource to ask questions, find answers, and discuss the novel.
Antonio offers up his body, a pound of flesh, as collateral should he default on his loan from Shylock. Antonio is using shylock's loan to help his friend Bassanio woo Portia.
This works into the theme of punishment and revenge. Portia is punishing Shylock for demanding a pound of flesh from a Christian. Shylock must pay with his religion for his pride and hubris over this court case.
The casket hides a prize, marriage to Portia, which various suitors must pick from.