Why a differentiation between 'betyr det' or 'det betyr'
The verb has to stay in the second place in a sentence. The "hva" acts as a conjunction and therefore a new sentence starts after that, so you have to put the subject first (det) followed by the verb in second place (betyr)
The other ordering is used for questions
Why is "mener" incorrect in this context?
å bety= to mean /"have as a meaning"; ¨å mene = to have the impression that, to believe, to intend, to think (about something)