"What do you wish for?"
Translation:Hvad ønsker du dig?
Duolingo is trying to tell us (by using "wish") that this is about gift-giving, as in "What do you want for Christmas". In Denmark we have a special phrase for this and since when we ask the question there is usually a holiday or a birthday coming up, so we don't feel the need to add the event in the phrase. We just say "Hvad ønsker du dig?". The reflexive word "dig" is how you can recognize it, because that's usually solely about presents. "Hvad ønsker du" can mean a lot of things, and - like the other replier wrote - it usually translates as "What do you want".
It could also be asked when someone makes a wish from a shooting star or the like.
But that usage would be much rarer.