The law of the prophets was a costly covenant between old-testament god and humanity. Or, you know, whatever subgroup was in charge of translations at the time.
The “fulfillment” mentioned is a single lump sum of holy lamb blood in place of the never-actually-complete exchange of not wearing blended fabrics, not getting tattoos, and sacrificing your firstborn on a rock in the mountains. And slaughtering prisoners of war who you tricked into getting circumcised as a condition of their surrender so that they would be vulnerable.

Matthew, particularly, was so obsessed with this idea that he even made up new scriptures that didn’t exist and said Jesus fulfilled them.