"What do you know about the mountain?"
Translation:Mit tudsz a hegyről?
You may include the te at the beginning or not. Either way is grammatically OK. If you include it, I think it does add a little emphasis to the "you", as in, "What do YOU know about the mountain?" Like maybe someone else has already said something about the mountain, and now I ask someone else, "What do YOU know?" - Te mit tudsz?
In conversation te (with a slight pause after it) can be a way of getting someone's attention at the beginning of a sentence. It's kind of like "hey, ..." or "hey, you..." It's fairly abrupt - about as polite as starting a sentence with "Hey" or "Hey you" in English. So I'd say be careful that you don't say it like, Te! Mit tudsz a hegyről? unless it's somebody you're on very informal terms with.