Picture this: a place where the streets are made of pure gold, gold as clear as glass. In this place there is no sickness, suffering, crying, pain or death. What a wonderful place! Perfect! This is how the Bible describes Heaven. God is in Heaven preparing a place for you, he wants you to one day live with him forever.
Perfect is how God intended Earth to be when he made it, but something got in the way. That thing is sin. Heaven will always be perfect, so one thing that cannot enter Heaven is sin.