Scholarly sources usually refer to sources created by scholars in a scholarly manner. Typically, this means that the article or book has gone through some variation of Peer Review and/or editing, in order to catch mistakes. It also assumes that the author has relevant expertise in the field they are writing in.
Figuring out if something is scholarly can be tricky, but there are a couple of rules that you can keep in mind:
There's no one easy way to figure this out, but keeping these general guidelines in mind can help. Many databases in the library also have an option to select only Scholarly or Peer-Reviewed items, so that is an excellent short cut.
Primary Sources are produced close to the event or person in question, from someone who has personally participated in or witnessed the thing they are writing about.
Secondary Sources are produced farther from the event or person in question, working from other people's information or writing after a large amount of time has passed.
These definitions can be tricky: often whether something is a Primary or Secondary source can depend on your topic, and sources can switch back and forth. It's often worth consulting with your professor or a librarian to determine whether something qualifies as one or the other.