Peer-Reviewed articles are sources that have been reviewed by experts in a specific field and determined to meet the expectations of that field for publication in specific scholarly journals. Not all journals use this process. And some peer-reviewed journals have sections that include non peer-reviewed articles. Resources on the open web, such as blogs, magazines, news sites, and general webpages are not the same as peer-reviewed articles.
In the peer-review process, scholars review manuscripts submitted for publication. The process is made as anonymous as possible. Manuscripts are reviewed and critiqued in the following areas: Journal needs/appropriateness, methodology (quality of research), citations/evidence/lit review, relevance to the field, etc. After review, manuscripts are either accepted, rejected, or returned to the author for revision and resubmission. This process can be involved and take many months (sometimes longer).
While this process generally guarantees accurate and relevant resources, you still need to use some critical thinking skills when searching the library databases for peer-reviewed articles. Keep in mind that in some fields, progress and advancements in understanding can happen rapidly (within a few years), so some resources can be out of date (think about medical fields). And also be aware that not everything in a peer-reviewed journal is necessarily peer-reviewed. Pay attention to the article details and the journal in which the article is published.
Your professor wants you to utilize empirical sources in your work. Empirical articles refer to sources that are reporting on original research. You may also hear these sources referred to as primary sources. Empirical sources will contain several sections, including a methods or methodology section. The methodology will tell you how the authors conducted their study or experiment, including how they selected participants, how they determined or selected survey instruments, and other processes.
Empirical sources can also include literature review, results, discussion, and suggestions for future research sections, in addition to the methods section. You can limit to empirical studies in the advanced search features of many psychological databases, including PsycArticles.
You may also discover and use secondary sources, like review articles - these articles will generally evaluate empirical sources. Some examples of review articles include literature reviews, systematic reviews, analysis papers, synthesis papers, etc. Unlike an empirical source, you won't see an experiment or survey tool described in the secondary sources. Review articles offer interpretation and commentary on the empirical source and its methodology (experiment/survey). This is the main way you can differentiate an empirical study from a review article.