Scholarly sources are written by scholars or researchers in a given field for other researchers and students in that field. They are typically published in scholarly journals. Scholarly articles can differ in structure across different disciplines, but there are some commonalities across all.
A popular sources is a source written for the general public. They are published in newspapers, magazines, blogs, etc. Keep in mind that professors can write for these publications, so looking at just author affiliation isn't enough to determine if a source is scholarly.
Professional publications can be a grey area between scholarly and popular sources. Check-in with your professor or a librarian if you are unsure how to count a source.