Subject headings are terms, chosen from a pre-existing list, that describe what an item contains or is about. They can be searched by selecting "Subject Heading Browse" in the "within" dropdown box. Subject headings are often made up of several components. For research on music history, the following information may be helpful:
FOR MUSIC AT A SPECIFIC TIME IN HISTORY, ENTER THE WORD MUSIC PLUS THE TIME PERIOD
FOR MUSIC AT A SPECIFIC PLACE, ENTER THE WORD MUSIC PLUS THE NAME OF THE COUNTRY
Keyword searches (using the "all fields" option in the drop-down box) can be useful if you're looking for books about a particular topic but you don't know the correct subject heading. For example, if you want books on music and the Counter Reformation, you could just enter Music Counter Reformation and then examine your list of results. Some will not be relevant, but for the ones that are, just click on their subject headings to see additional resources.