Where is the italicization in the King James translation of the Bible from and what does it mean? Take, for example, Ecclesiastes 9:4-6 in the King James version:


  For to him that is joined to all the living there is hope: for a living dog is better than a dead lion. For the living know that they shall die: but the dead know not any thing, neither have they any more a reward; for the memory of them is forgotten. Also their love, and their hatred, and their envy, is now perished; neither have they any more a portion for ever in any thing that is done under the sun.


Why is the second occurrence of the word 'thing' italicised?

Here's another example, Matthew 22:41-46:


  While the Pharisees were gathered together, Jesus asked them, saying, What think ye of Christ? whose son is he? They say unto him, The Son of David. He saith unto them, How then doth David in spirit call him Lord, saying, The Lord said unto my Lord, Sit thou on my right hand, till I make thine enemies thy footstool? If David then call him Lord, how is he his son? And no man was able to answer him a word, neither durst any man from that day forth ask him any more questions.


'The Son', the second occurrence of 'man', and 'questions' - why are they italicized?
The italics indicate a word that is implied in the Hebrew or Greek. Although some people describe these as "added" words, that is not really a useful way of thinking of them. Yes, there is often ambiguity in what is implied, but there's just as much ambiguity in the explicit words. Every Bible translation accounts for these implied words, but most do not put them in italics, because that is just how translation works.

An example of implied meaning in English is when we talk about "the rich" and "the poor". We understand that those mean "the rich people" and "the poor people". There would be many languages where translating those phrases would necessitate explicitly stating that it is people we're talking about.