* Boolean searching is the process of combining search terms with "OR," "AND," and "NOT." Discovery Search supports Boolean searching multiple ways. By default, multiple search terms are considered to have an and between each word. Index labels combined with Boolean operators can be entered in any search box, including the multiple search boxes on the Advanced Search screen. Boolean searches are processed in left to right order.
Using AND will reduce the number of search results. You can instead search using or (to broaden your search results) by inserting OR in all capitol letters between words. You can exclude search terms using NOT in front of a search term. Words and operators can be grouped using parentheses, leading to very complex searches. The New York Public library has a nice visual explanation of using the boolean operators.
* In any case, Boolean operators must be capitalized to ensure they are recognized (i.e., "OR," "AND," and "NOT").