What is a Narrower Term?
Library catalogs and specialized article databases will often create a "hierarchy" for categorizing the words and phrases they use in the database. As the name implies, narrower terms are those that include a smaller subset of the full database. Narrower terms are most useful when you are finding more things related to your topic than you can really deal with. Narrower terms allow you to be more "precise" in your retrieval.
For example, narrower terms for endangered species might include "endangered animals." Or, narrower yet, you might want to look at names of individual species on the list, such as "Spotted Owl" or "Bald Eagle." Related terms for these would include the scientific names for each of these species.