In Norwegian does "Hva betyr brevet?" ask "What does the letter say?", asking for a summary or translation?
In English, "What does X mean?" works for words, sentences and even multiple sentence messages, but for longer text, like letters or books, I don't think it would be used. I'm pretty sure most native speakers would instead ask "what does the letter say" and they'd interpret "what does the letter mean" as "What does the fact that you got the letter mean".
A good question, had to google it. I do know that in swedish they say: 'Vad betyder det?' for 'Hva betyr det?' and 'Vad menar du?' for 'What do you mean?' It appears that in norwegian, it goes: 'Hva mener du (med det)?' for 'What do you mean (by that)?' So, 'hva betyr du' is not equivalent to 'what do you mean?' if that is what you wondered. Instead, I believe it would be asking to define the word 'du'.