Subject headings are a formalized way of of searching library catalogs. The right subect heading can help you get to information more quickly.
For example, the American Revolutionary War can be described using the following terms:
    American Revolution
    American Revolutionary War
    War of the American Revolution
    American War of Independence
    War of American Independence
    War of Independence
You could search the library catalog for all those keywords. Or, use one subject heading to find all of the library's materials about the the American Revolutionary War:
Nordamerikanischer Unabhängigkeitskrieg
Find books on your topic by using a keyword search to find relevant subject headings. For example, search the keyword "Nordamerikanischer Unabhängigkeitskrieg", in the library catalog. By clicking on one of the keywords for the first book, Die amerikanische Revolution, the subject headings lead to other relevant books on this topic.