Essesntially, a thesaurus organizing words and phrases, intended to cover all the topics that library holdings ( books, periodicals, recordings, etc.) might be classified under. The LCSH was developed by the Library of Congress. It is a controlled vocabulary originally designed for use by the LoC for subject indexing in their catalog, but has been used by many other libraries and even some online bibliogrphies. Because libraries that use the Library of Congress Classification System to organize their books do subject indexing according to LCSH, when you do a subject search in those libraries, you will have the best luck if you search on words/phrases that are entries in the LCSH.

This is the main Library of Congress site that deals with LCSH: