Translation:What is he reading on her tablet?
it's a lovely word, that flies off the tongue with sharp cracks of the tongue. If that's not the case for you, divide it up into it's component parts Nett (web, both as in spider's web, world wide web, and fisherman's...net) and Brett (board, both tablets, and snow/surf/skate boards)
The question is asking: What is (some guy) reading on (some other female's) tablet? It should not be confusing, because in the Norwegian language, when you refer to something that is someone's own property, you would use 'sin' or 'sine.' Thus, if they use 'hans,' 'hams,' or 'hennes' instead of sin or sine, you know it is referring to some second (other) person or party.