Keyword searches look for search terms anywhere in the record, i.e. the author, subject, title fields, etc. When you find a work of interest, check the full view of the record to see subject headings that can lead to further relevant works.
Subject headings are categories--each standing for a particular event, concept, person, etc.--that allow you to focus your search more precisely in Franklin. Use the Subject Heading Keyword option to search and the Subject Heading Browse option to browse a list.