To fulfill the law is a rabbinic technical term used to refer to demonstrating a full understanding of the law by words or by actions. As Jesus said he came to do, he proceeded to fulfill the law six times in the rest of Matthew 5 by instructing how to understand and obey it, and he fulfilled the law by giving us a perfect example to follow of how to obey it. Love is the essence of the law and Jesus summarized it as being about how to love God and how to love your neighbor, so by doing things in obedience to the law that demonstrate love for you neighbor you are fulfilling the law by showing that you understand how to obey it. Every Sabbath in a Synagogue a rabbi would take a Torah scroll to Moses' seat and fulfill the law by interpreting and explaining how to understand it. Anyone and everyone can fulfill the law countless times throughout their life, so it does not refer to obeying the law perfectly, but to obeying it correctly. Our flesh prevents us from obeying the law perfectly, but with God nothing is impossible, so we can be made be like Christ in having perfect obedience to it so that we might meet its righteous requirement (Romans 8:4). That will be the case when He who began a good work in us is faithful to complete it on the day of Christ Jesus.