Tetelestai is in the perfect passive indicative form of the verb. It means that something is completed with enduring effect or state.

If Jesus said this in Aramaic, we can discern the Hebrew means of stating it, which is close to the Aramaic, if not exact. The same translatable meaning is possible. It would have to be in the Pu’al stem, perfective form, which is the passive form of the factitive stem, which stem means “to have been put into a continuous state of.”

In fact, the Semitic translation may be more powerful than the Greek. If we properly translate the Pu’al stem, not just the word, Jesus would have said, “It is [has been] put into a continuous state of being accomplished.” The Hebrew is KuLLeTaH. Or if, as one has mentioned, Jesus stated it more as an exclamation, as I believe He did, it would be preceded by HiNNeH. This is commonly translated as “Behold.” It is an emphatic particle that emphasizes the immediacy both of time and of location, and should be translated more literally: Here and Now!

So Jesus may have said: “Here and now it is [has been] put into a continuous state of being accomplished!”

Regarding what “it” is, it is either one foundational aspect of the atonement from which all other aspects come forth, or, as Thayer says, it is everything regarding what the Father placed upon the Son to complete. This is why I put the subject of the verb as feminine, which in Hebrew is used to refer to more abstract meaning, broader/detailed application, or a previous grammatical statement or idea (among other things) which is the antecedent of the of the pronoun.