Subject headings are created by the Library of Congress to make sources discoverable. They're like hashtags but for books and articles, and unlike hashtags they're maintained by a central authority, making them consistent across platforms.
For this seminar, the most useful subject heading is Vietnam War, 1961-1975, though in rare instances you may see Vietnam Conflict or Vietnamese War instead.
Because subject headings are hierarchical, you'll often see subheadings that narrow a topic's scope. For example, here are some of the subject headings for the book Reporting Vietnam (via WorldCat).
Using these constructions in a subject search will yield other books on the topic of journalism about the Vietnam War. You can use the same process to locate materials on whichever topic you choose. The key is to begin with one useful text, then use its subject headings to lead you to related resources.