What is the meaning of INRI on the cross above Jesus' head?

INRI is an acronym of the Latin inscription 'IESVS·NAZARENVS·REX·IVDÆORVM' (Jesus Nazarenus, Rex Iudaeorum), which translates to English as "Jesus of Nazareth, King of the Jews." 

This was written as the "crime" that Jesus had committed on the top of the Cross and is mentioned in John 19: 19, although it was not just written in Latin (as the Romans, who spoke Latin, were the executioners), but we are told that it was also written in Hebrew (for the Jewish people to read) and Greek (which was the universal language of the whole Mediterranean area at that time - as there were many foreign visitors in Jerusalem at that time for the Feast of the Passover.

John 19: 19-22: 'Pilate wrote a title also and put it on the torture stake. It was written: "Jesus the Naz·a·rene´ the King of the Jews." 

Therefore many of the Jews read this title, because the place where Jesus was impaled was near the city; and it was written in Hebrew, in Latin, in Greek. However, the chief priests of the Jews began to say to Pilate: "Do not write 'The King of the Jews,' but that he said, 'I am King of the Jews.'" Pilate answered: "What I have written I have written."

Latin became the Language of the Roman Catholic Church. But, in Latin, they used the "I" for the Letter "J" (both in Jesus and Jews), their word for King was "REX", and they used the "V" for the "U" . So the sign would have been something like this one

IESVS NAZEREVS, REX IUDEA, Or using the first letters of each words: INRI

The final language on the sign was Hebrew which is the Jewish language. Now the first word wasYehoshuah (Jesus). The first letter of that is the smallest letter in the Hebrew alphabet. It is the little yod, which looks like an apostrophe. (Hebrew is read from right to left.) Yehoshuah and according to the Bible it was "Jesus the Nazarene." The next word was ha Nazarite. The next word would be "and," which is what we call a wha. It is like a little line with a hook on top. The next one would be the king of Jews Hamelech Yudio. What you had on the cross reading from right to left in the first letter, are the four letters of the Tetragrammaton YHWH. YHWH is the covenant name of God in the Old Testament Hebrew which did not have vowels (today we know this as Yahweh). In John 19:19-22 the chief Priests wanted Pilate to take the sign down because the sign recognized Jesus as Yahweh or God.