That Jesus' blood paid all the penalty our sins threw upon us. We go to heaven by looking to the blood of Jesus. Not our good works. In fact that only way to get to Heaven is by Jesus' death on the cross, and giving our lives to Him to no longer live a sinful life with the rest of this world. Many things made from wood covered in gold.
Wood is perishable -organic.
But mercy seat is solid gold. Totally God's work. We can never do the work to get us to God. God did the work. The work of Jesus' death, shedding His blood saved us.