Subject headings constitute a controlled vocabulary that can help you find sources by their major subjects. The Library of Congress maintains this vocabulary, ensuring a standard implementation in library catalogs, scholarly databases, or wherever LC subject headings are used.
Imagine you're developing a topic on theme parks. You search that keyword in the library catalog and find the following book.
Now look closely at the book's item record. You'll see a list of, you guessed it, subject headings! These supply a broad outline or representation of this book's contents.
Each of these headings can be clicked to return all books thus tagged, making subject headings a powerful method for discovering relevant resources. And take note of the compound structure; subject headings can be narrowed to subtopics. One of the subject headings in the example above will help us find catalogs about the design of amusement parks.
Caveat searcher! Searching only by subject headings may inadvertently filter relevant results. It's best to begin with broad, keyword-based searches, then search by subject when you have a well defined sense of the resources you need. Research is often a process of moving between broad and narrow levels of inquiry.