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.
Look at the most relevant 2 or 3 results towards the top of the list.
Click into the title of each of those results, and then click "View Description."
Scroll down to the "Subjects." You will see these are active links that you can click to find more resources about the same topic.