Peer-reviewed resources have been vetted by experts within the field. Work is submitted to a journal, peer-reviewed, returned with notes and edit suggestions, then re-submitted and either accepted or rejected. Because of this process, these articles are more trusted. When your professor says scholarly, peer-reviewed resources this is what they mean.
On this page there are recommended databases and journals that provide peer-reviewed content. There are filters to help you with this process, but it is always best to double check that a journal is actually peer-reviewed. Go directly to that journal's website and they will tell you.
Popular resources are more general than others. They have a wider target audience, use simpler language, and are not published in academic journals.
Examples are:
Reference materials provide overviews and quick facts on topics. Often, they have an index.
These materials include your textbooks and encyclopedias. On occasion, these can include government publications.