How to add Author to library? What is the purpose of Topic?

Pardon my  brain fog.  An new to this stuff and quite lost

I did online training months back and now need to organize references this weekend. 

Have my library orgainzed into Groups (and subgroups)   But also categories with Authors and Topics  

I don’t know how to add to authors and don’t know what the purpose of topics is as opposed to subgroups. All I can find is about managing groups

Thanks for help!