A keyword search is a very broad search.
This type of search looks for the word ("dating, " for example) in the title field, author field, and everywhere else in the record of an article or book. It also looks for every possible meaning of the word. Here are some results you would get from a keyword search:
A search for "dating" will bring you back a book called "Sex, dating, and delinquency: Do precocious relationships lead to crime?" That search will also have geology articles such as "Archaeological dating of prehistoric stones found in North America"