It means it's not subject to discussion. It's a very categorical insistence that whatever it is they are talking about is going to remain the way it is, and no suggestions can be made to change it in any way.
"Out of the question" can also mean that something is too impractical even to consider. Like, say, you are 25 miles away from your home and your car broke down. You might think to yourself, "Well, walking is out of the question... "
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