Right before dying, Jesus famously declared, “It is finished,” (John 19:30). The meaning of this statement, and by extension the cross, is twofold. First, the immediate earthly life of Jesus was over. He would show Himself briefly after the resurrection, but the cross marked a turning point in His relationship to other people.
Second, the ultimate punishment for sin was and is complete. No amount of prayers or good deeds could ever make us more fit for Heaven than what Jesus did and completed on His own. The cross represents that our punishment has been taken, and Jesus stands ready to accept and forgive us if we only ask.