“The italicized words in the King James Bible are words that were added by the translators to help the reader. This is usually necessary when translating from one language to another because word meanings and idioms change. So, to produce a more readable translation, the King James translators (1604- 1611) added certain words to the Bible text. However, to make sure that everyone understood that these words were not in the available manuscripts they set them in italics.”
from here