Peer-reviewed resources have been vetted by experts within the field. Work is submitted to a journal, 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 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.
When searching, filter by choosing Scholarly (Peer Reviewed) Journals.
The journal title is listed next to Source. Click the journals page and it will tell you whether or not it is a peer-reviewed.
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.