Searching by subject allows you to find sources that may not have turned up with your keyword search. A keyword search looks for word-for-word matches of the terms you typed in. Subjects are a controlled vocabulary applied to each item in the library catalog that describe what the source is about.
One of the best ways to search by subject is to start with a keyword search.
Indians of North America -- First contact with other peoples
Indians of North America -- History -- 18th century
Indians of North America -- Economic conditions;
Indians of North America -- Women; Navajo Indians