Subject (or topic) is one of the aspects of an item you can use to perform or refine a search in the Library catalogue (in Library speech, a "facet"). Every item in the catalogue has several subjects chosen from a list of controlled vocabulary (the Library of Congress Subject Headings) to help users retrieve the best results for their searches. You do not need to use those specific terms to find items relevant to your study or research: if it does not recognise it as a subject heading, the system will simply use the term as a keyword instead.