Many wiser heads than our have argued that out..
So what do you think IT is ??
One of the funnier (unwittingly so) Christian T-shirts I've seen was a t-shirt with a picture of Jesus on the cross and the greek word "teleo" above it. Unfortunately, teleo is the present first person active of what Jesus actually said "tetelestai." So the t-shirt said "I am finishing." Just enough knowledge to be deadly.
"It" does not need to be tied to one definite meaning. There are a multitude of suggestions which are applicable and Biblical. "It" could be Jesus struggle through the ordeal of death. I would not suggest this as an all-inclusive meaning, but it certainly could have been one thing He meant. "It" must at some level include God's plan of salvation for mankind and the fact that Jesus death (and proleptically his Resurrection) accomplished available salvation and wholeness for humankind (or a couple of people who got voted for, either way :-) ).
"Tt" would have to be something that was not "finished since the beginning of time", but something that was just finished. He had just accepted some sour wine.
It was the completion of the Passover Seder.