Subject headings are predefined terms that the Library of Congress assigns to printed material. Subject Headings use a precise, controlled vocabulary that typically cannot include multi-disciplinary or complex topics.
You'll see these listed as "Subject" at the bottom of each catalog record. Below are some examples:
Women political prisoners -- Soviet Union -- Biography
Soviet Union -- History -- 1925-1953
Europe, Eastern -- Economic conditions -- 1945-
Quality of life -- Soviet Union
Soviet Union -- Economic conditions -- 1975-1985
Soviet Union -- Politics and government -- 1953-1985
Soviet Union -- Social conditions -- 1970-1991
Sports -- Soviet Union -- History
Physical education and training -- Soviet Union -- History
Domestic relations -- Soviet Union -- History
Family policy -- Soviet Union -- History
Church and state -- Soviet Union