"...then what does fulfill mean?"
It is a covenant thing. It is like making the last payment on a purchase agreement. It is fulfilled.
Heck, according to Dictionary.com:
1. to carry out, or bring to realization, as a prophecy or promise.
2. to perform or do, as duty; obey or follow, as commands.
3. to satisfy (requirements, obligations, etc.): a book that fulfills a long-felt need.
4. to bring to an end; finish or complete, as a period of time: He felt that life was over when one had fulfilled his threescore years and ten.
Are you not overlooking that Christ said he was not come to destroy the LAW or the PROPHETS??? The Prophets are the ones who did the penning of foretelling of His coming and Christ continually quotes the Prophets. He had much to say about Moses who is described as the LAW giver as well as a prophet.