If something (an article, a PhD thesis, or other resource) has been peer reviewed it means that the work has been examined by other professionals in that same field - either researchers or practitioners - who have then fed back to the author any recommendations for improving the quality of the work, as well as highlighting any inaccuracies that will need correction before publication can take place.
Peer review takes place across a wide range of disciplines and you can find an open access article about it from a scientific perspective on PubMed Central.
Kelly, J., Sadeghieh, T. & Adeli, K. (2014) 'Peer review in scientific publications: benefits, critiques, & a survival guide', The Journal of the International Federation of Clinical Chemistry and Laboratory Medicine, 25(3), pp.227-243.
