"What can I do for you?"
Translation:Wat kan ik voor jou betekenen?
I think your answer is the correct one. To "beteken" means to "mean" something - e.g. "Wat betekent dat?" = "What does that mean?" Perhaps the following scenario might make sense of the given translation: You're at a party and get into a conversation with someone you've never met before. Although you can't quite figure out why, you get the feeling that that person wants to know you for a reason, and you wonder "What significance could knowing me have for this person?", or "What would I mean to this person?" or "What could I do for this person?" In Dutch - "Wat zou ik voor dit persoon betekenen?" That's my best guess.
Lidor is right. "Betekenen" in this sentence is "to be of value/importance/significance/meaning" to someone (note the similarity between English and Dutch: word "sign" in significance and "teken" in betekenis). "Wat kan ik voor jou betekenen?" could be said by a waiter as "Can I take your order?". It is also used in situations of grief or people in need, often: "Kan ik iets voor je betekenen?" meaning "Is there anything I can do to help or relieve your troubles?"
In daily life we translate this sentence to "Wat kan ik voor je doen".
"Wat kan ik voor jou/je/u betekenen" is more formal Dutch. Here is the link to a few examples in context: