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Peer reviewed means that professionals in the field evaluated the source before it was published. Unlike popular sources (such as books, magazines, newspapers, etc.) which are written for a general audience, peer reviewed sources are written by professionals, for professionals.

For example, a peer reviewed source on a topic related to mechanical engineering is meant to be read by other people in the field of mechanical engineering, like professors, students, and other mechanical engineers.

Peer reviewed journals are well respected in the academic community. These are peer reviewed (sometimes called "refereed") to ensure the quality of the research articles in the publications. Most of these journals are published by reputable publishers or university presses. Rigorous criteria must be met before a peer reviewed source is published, so a peer reviewed source is generally better quality than a popular source.