Jesus faced excruciating and prolonged pain and suffering on the cross. The suffering lasted pretty long, but it eventually came to an end, for Jesus died from it. Right before Jesus died, He proclaimed a big declaration -- "It is finished!" (19:30)
There are many things that Jesus referred the "it" to when Jesus said the famous saying.
Jesus's physical life was finished on the cross. On the cross, Jesus was officially executed. He was left to die for a long time and faced extreme thirst.
The penalty of sin was finished on the cross. According to Hebrews 9, the Hebrew law requires that shedding of blood is required for atonement. Despite being innocent, Jesus died as if he was a criminal. In other words, he took on the penalty of our sin for our atonement based on Hebrew law. The substitution is part of what is known as the "great exchange".
Jesus's job was finished on the cross. We've talked about how Jesus paid the penalty of sin as per Hebrew law one bullet point ago, and that act was Jesus's big calling from the Father.
Because Jesus paved the way for atonement (as per Hebrew law), various powers from the side effects of sin (like guilt and shame) were finished on the cross. Looking to Jesus for recovery from bad mistakes because of what He did is super effective.