You can think of peer review as a "stamp of approval" from academic experts. When an article is published in a peer-reviewed journal, you can be certain that experts in the relevant field have read it and, independent of their own particular opinions, verified it to meet a high standard of scholarship.
Scholars rely on peer review to ensure that the scholarship they exchange with each other is always based in good research and the established standards of their discipline.
The peer review system is similar to quality control systems that you see in everyday life. Just as you might be reassured to see a Health Department certificate in the window of a restaurant or a "Verisign" logo on a website that requires you to enter sensitive information, the peer-review system provides an efficient standard of trustworthiness in academic scholarship.
If an article has been published in a peer-reviewed journal or by a reputable book publisher (such as a university press), you can assume that: