Most databases by default search for keywords found anywhere in the document. If you were to enter the term:
without any qualifiers or delimiters, the search results would include
Stevens, Timothy. (2012). Christopher Columbus : a biography. London: Notting Hill Press
as well as:
Hill, Christopher (1985). Arrest of Ships. London : Lloyd's of London Press Ltd. , 1985
And if you were searching a database of full-text documents, depending on search engine defaults, your search terms might pull up a document that has the word "Christopher" on page 4, and the word "Hill" on page 444. Your results would be "comprehensive" in catching all documents with your terms used anywhere, but they would contain a great deal of materials that are not "relevant" to what you are looking for. In the library Discovery Service, a search this indiscriminant could retrieve tens or even hundreds of thousands of documents -- far too much for you to wade through in search of items useful to your research.