While a book's table of contents give you an overview of what it contains, the index gives you a more complete listing of the topics covered and directs you to the pages on which each topic is mentioned or discussed. Using the same principle a periodical index looks at the contents of many different magazines and journals and refers users to specific issues with articles on the subjects listed.
Periodical indexes come in different flavors.
BUT . . . Since there are hundreds of thousands of periodicals, Reader's Guide clearly won't cover all your research needs. As a matter of fact, for most of your course work you are likely to need to look for more in-depth information than you will find in Reader's Guide . That's because Reader's Guide is a general interest index . So, finding an index that covers your topic is the first step to finding the articles you need.