A subject heading is a special type of category that groups together materials in the Library that share a topic, author, discipline, or particular format. While simple keyword searches are a good start for finding materials, a subject heading search is an advanced type of search that often produces much more precise and targeted results that are often exactly what you need.
The subject headings used in the Library catalog are standardized Library of Congress headings, which may be subdivided by geographic area, chronological period, genre, or sub-topic. The language of subject headings is not particularly intuitive or natural, so you shouldn’t hesitate to ask a librarian for help in finding the correct subject headings.
A good way to find catalog subject headings for your topic is to do a keyword search in the online catalog. For example, let's say you look for "Maxine Hong Kingston" and you see the following record in your results:
Look in the "Topics" field: these listings are the subject headings for this book, such as "Kingston, Maxine Hong - Criticism and interpretation." The subject headings in the online catalog are hyperlinked, so simply click on the heading you're interested in to search for books under that heading.
It's best to use both fairly broad headings as well as specific ones, in order to make sure you don't miss any relevant material. You'll likely find multiple headings that are relevant to your topic, and you will find a variety of resources if you look at all of them.