In secondary sources, authors analyze and interpret primary source materials.
Secondary sources can be scholarly or popular. Scholarly sources (sometimes called "academic" or "peer-reviewed" sources) are written by and for experts and typically include bibliographies and citations. Popular sources are written for a general, non-expert audience and can be authored by anyone.
Another useful feature of Google Scholar is its ability to allow for easily finding articles which have cited an article that you have found.
Step 1: When looking at search results, check for the 'Cited by X' link underneath each result. That will tell you how many subsequent articles (that Google Scholar is aware of...it's not 100% comprehensive! This is a ballpark figure) have cited this particular article.