・・・。
This kitchen knife... was made by a famous swordsmith.
Literal
This kitchen-knife... famous sword-craftsman [subject-が] made thing [copula-な][explanatory-んです].
刀匠 is 'swordsmith' — a prestigious traditional craft in Japan. That a 包丁 (kitchen knife) was made by a 刀匠 (traditionally associated with katana) underlines the knife's quality and pedigree. ものなんです uses the nominalizer もの for tangible things + explanatory なんです to reveal new information with a touch of pride.