Like most academic libraries, the GW Libraries uses Library of Congress Subject Headings (LCSH) to assign specialized descriptions to individual items. A Subject search for the term "London" would pull up items about the city. In contrast, a Keyword search for the term "London" would pull up items about the city, as well as items published in London or by an author named London which may not be useful.
A librarian can help you identify subject headings useful to your search, but as you become familiar with research you may notice the logical rules by which LCSH are created. (Search term of one or more than one word, national subdivision, temporal subdivision, subtopic, etc.) For example:
Women--Afghanistan--Social life and customs
Women--Afghanistan--Kabul--Social life and customs--21st century
*Summarized from Houston, Natalie, "Using LOC subject headings," ProfHacker (blog), The Chronicle of Higher Education, October 23, 2009 (10 a.m.), http://chronicle.com/blogs/profhacker/using-loc-subject-headings/22785.