"It is finished" is the English translation of the Greek word Tetelestai, which was the last thing Jesus' said before dying on the cross. Tetelestai comes from the verb teleo, which means "to bring to an end, to complete, to accomplish." It's a crucial word because it signifies the successful end to a particular course of action. It's the word you would use when you climb to the peak of Mt. Everest; it's the word you would use when you turn in the final copy of your dissertation; it's the word you would use when you make the final payment on your new car; it's the word you use when you cross the finish line of your first 10K run. The word means more than just "I survived." It means "I did exactly what I set out to do."

But there's more here than the verb itself. Tetelestai is in the perfect tense in Greek. That's significant because the perfect tense speaks of an action which has been completed in the past with results continuing into the present. It's different from the past tense which looks back to an event and says, "This happened." The perfect tense adds the idea that "This happened and it is still in effect today."

Note one other fact. He did not say, "I am finished," for that would imply that he died defeated and exhausted. Rather, he cried out "It is finished," meaning "I successfully completed the work I came to do."

What was finished:
The ESV Study Bible comments,
"It is finished proclaims that all the work the Father had sent him to accomplish (cf. 4:34; 9:4) was now completed, particularly his work of bearing the penalty for sins. This means there was no more penalty left to be paid for sins, for all Jesus' suffering was 'finished' (see Heb. 1:3; 9:11-12, 25-28)."
There is NOTHING left that WE can do, except COME TO HIM IN FAITH. We cannot work our way to heaven. Accept Him alone as your personal Savior and live for Him.