The letters "INRI" are initials for the Latin title that Pontius Pilate had written over the head of Jesus Christ on the cross (John 19:19). Latin was the official language of the Roman Empire.
The words were "Iesus Nazarenvs Rex Ivdaeorvm." Latin uses "I" instead of the English "J", and "V" instead of "U" (i.e., Jesus Nazarenus Rex Judaeorum). The English translation is "Jesus of Nazareth, the King of the Jews."