AND is used to narrow down your search. When you add AND between two or more search terms, your search results will include all of your search terms.
The blue area in the illustration to the right represents the results that would be returned for the search Pacific AND Whale. All the articles will have both terms.
Lets take a closer look.
In the database, Web of Science, the searches for Pacific, Whale, and Pacific AND Whale break down like this:
Pacific = 89,154 results
Whale = 6,321 results
Pacific AND Whale = 541 results
Take note that many databases automatically insert AND between the terms you enter into the search box.
This is a good thing to keep in mind, since the more terms you enter, the narrower your search will be - which means you'll get fewer results.