Using "AND," "OR," can either broaden or narrow your search.
Connecting one term to another, such as "lobster industry" AND "Maine" means that instead of searching for anything on the lobster industry, or anything that mentions Maine, you're only searching sources that discuss both of these topics.
Using "OR" broadens your search. This is often helpful when you're researching a topic that has different terms with similar meanings.
In the example below, the advanced search in LibrarySearch has been selected and the search is for "lobster populations AND climate change" OR "lobster populations AND global warming OR "lobster populations AND ocean temperatures." So this search will look for any sources that discuss lobster populations and use the terms climate change, global warming, or ocean temperatures.