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."
When Jesus cried out "It is finished," he meant "It was finished in the past, it is still finished in the present, and it will remain finished in the future."
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."
Tetelestai, then, is the Savior's final cry of victory. When he died, he left no unfinished business behind. When he said, "It is finished," he was speaking the truth.