Scholarly books and articles are "scholarly" because they have gone through a vetting process called peer review. The peer review process basically happens like this:
Why is this important for you?
The information in a scholarly text is more reliable and credible
than information in popular sources like newspapers and magazines.
Also, scholarly books and articles tell you a lot about common arguments and ways of thinking used by scholars in a particular discipline.When you are writing a paper or giving a presentation on a topic in that discipline, it is helpful for you to be aware of those arguments and ways of thinking. To help evaluate a scholarly publication from a non-scholarly, look at this great chart from Western Carolina University.