In simple terms, peer review is a quality control process applied to an author's scholarly work and is designed to ensure that published works are valid, original and of high quality.
The terms "peer reviewed" and "refereed" mean the same thing. That is, the work has been subjected to a quality control review by experts in the relevant field of research.
When an authors research has undergone the peer review process and been published in a journal, the published article can be called a "peer-reviewed article". When a journal publisher applies the peer review process to all articles they publish, then the journal can be referred to as a "peer reviewed journal".
At university you sometimes hear about "scholarly" or "academic" works and these terms may be used to mean "peer reviewed". It is recommended that you seek clarification as not all scholarly or academic works have undergone a formal peer review process.
There can be debate in academic circles about whether peer review is useful and whether it adds any value to the academic and research process. Here are some opinions that students should be aware of.