The chain is your sin. Your sin holds you captive, and you can't escape. Every time you break God's law, every time you are selfish, cruel, have hate or bitterness. Every time you know something is wrong and do it anyway. Those sins have you in bondage.
The nail is the price required for sin. It's a symbol of death which Jesus paid for you. He took the nails into His wrists to take the penalty for your sin. He died so you don't have too suffer for eternity.
The key is your faith. It's trusting in Jesus, it's opening your heart and life to Jesus. It's when you stop trying to get it right on your own, when you stop trying to find the answers on your own. It's the key to eternal life, the key to filling the empty place in your heart. It's the key to freedom.
The heart is your new life in Christ. When you turn from your life of self rule and seek after Jesus, He gives you a new heart and a new life. When you pray and ask Jesus to come into your heart, He does and He will clean house.
The top and bottom are heaven and hell. Heaven is eternity with God, Hell is eternity seperated from God because of your sin. Where will you be?
