It means that someone is examining and explaining things that are the same and the things that are different between two similar things. It would be like taking two different kinds of flowers and noticing the differences in things like color, shape, etc, and also the similarities such as petals, stems, etc.
A because "Because she was too young" can't stand by itself. To test if it is dependent, see if it makes sense by itself. if it does, it is independent.
