Any source that's likely to be of interest to a lot of people, not just other experts or scholars, is considered a popular source. A popular source could be a book, website, newspaper article, or magazine article.
Popular sources are never scholarly or peer-reviewed. In fact, sometimes popular sources are referred to as non-scholarly sources. However they could still be credible sources. For example, most people would agree that fs.usda.gov (i.e. The US Forest Service) would provide credible information on their site.
Some popular sources, such as website, are difficult to evaluate for credibility. If you have questions, ask for advice from your instructor or a librarian.
