If you find a helpful article, it's a good idea to look at the sources that article cites to identify other, earlier articles that may be relevant to your research.
A citation index allows you to find later articles (and in some cases book chapters) that cite an earlier article. In other words, if you find an article you are very interested in, you can look up the article in a citation index to identify later articles that list that original article in their references. These articles will often be related to the topic of the original article; a citation index can lead you to newer articles on a related topic.
Google Scholar is one of the easiest tools to use as a citation index. Note: If you aren't authenticated through MyLSU, Google Scholar will not recognize you as being affiliated with the university, and these articles won't be available as full text directly in Google Scholar.