AND is a Boolean operator that narrows your results to be more specific.
NOT is another Boolean operator that will narrow your results.
OR expands the number of results you will get. When you use OR to connect two keywords, your article results will include some articles with only your first keyword, some articles with only your second keyword, and some articles with both keywords. In the example below for vibaphone OR vibraharp, some articles will include vibraphone, some will include vibraharp, and some will include both terms.
Different articles use different synonyms, or words with same meaning.
Another way to expand your results is called truncation. You can search for multiple words at the same time that start with the same letters by placing a * after the letters.
Here are some examples: