I would take this question at face value - it is asking what wing dihedral
is, not its
effect. The wikipedia reference answers the former and (maybe unhelpfully?) expands on it to explain the latter. Look up anhedral for the reverse.
Just a thought. As for ATPL preparation, you will need a first class degree(with Hons.) in second guessing and bull!!!! before even contemplating opening that shiny package from BGS
