You don't need to know it all and asking questions will not only improve your understanding of the role, it will also show you are keen to find out more. So if you are asked what interests you about the job (and you almost certainly will be), don't be afraid of saying something like, "Well from your advert it seems like this job will offer me a good chance to develop my skills and understanding within this industry, but perhaps you could give me some more information about exactly what the position will involve." You've answered their question but at the same time you are getting more information for yourself.