Library of Congress Subject Headings (LCSH) are assigned, perhaps unsurprisingly, by the Library of Congress. For more about Library of Congress Subject Headings click here.
For our immediate purposes, subject headings are useful in same way that categories are when shopping for something online: subject headings get you exactly want you want. Every single book published is assigned subject headings by catalogers, so that if you click on a subject heading in a computer catalog it will pull up every other book that has been tagged in the same way. Rather than throwing words into a search and hoping for the best, subject headings allow you find materials that are related to your topic but not be using the same words you are. Using subject headings will greatly increase the depth of your search and ensure that your literature search is complete.
In the case of this subject heading: