There are quite a few English words that contain letters that are not suppose to be said including: debt and write. These left out letters can often tell us about the history and origin of the word and some places in the world has these which can tell us about who lived there at the time. This is called etymology.
Words over time changed from what they might have been. Ancient people could have said the word thumb with the b sound. But in current English we do not say it and this goes for other words as well. The word know has a silent k and w. It sounds like you are saying the word "no". There are other words that have silent letters but we do not know for sure why those silent letters are not said.
The significance of this quote is Montag's realization of what his job is actually about and what it implies. He became a fireman because he was meant to. However, he is now haunted by this new feeling and does not know anymore what is right and what is wrong.
