Jesus tells us in John 17:4, "I have glorified thee on the earth: I have finished the work which thou gavest me to do." John 19:30 reads, "When, therefore, he had received the vinegar, Jesus said—It is finished! And, bowing his head, delivered up his spirit."
From John 17:4, we learn that Jesus came to earth to fulfill God's desire, his plan. He left his heavenly existence. Hebrews 2:9 gives us further insight. It reads, "But we see Jesus, who was made a little lower than the angels (as a man) for the suffering of death, crowned with glory and honor; that he by the grace of God should taste death for every man." Jesus’ work was to ransom all mankind from death. As a man, Jesus became our mediator. 1 Timothy 2:5,6, "For there is one God, and one mediator between God and men, the man Christ Jesus; Who gave himself a ransom for all, to be testified in due time."
Original man, Adam, had disobeyed God and sinned. Adam was created perfect. Through his disobedience he, and all of mankind, fell from their created relationship with God. In order for this relationship to be reestablished, the penalty for sin had to be paid. Jesus voluntarily gave his life to pay this penalty for sin, to do his Father's will. Hebrews 10:7, “Then said I, Lo, I come (in the volume of the book it is written of me,) to do thy will, O God.” His death by crucifixion, the ransom sacrifice, was the price paid for man's sin.
When Jesus declared “It is finished,” he was affirming that he had willingly, voluntarily fulfilled God's design and finished his earthly, consecrated walk through his sacrifice on the cross at Calvary. He had “tasted death for every man”. He had fully paid the ransom price – a perfect man, Jesus, for a perfect man, Adam. This is what was finished.