Peer review is the certification process by which academic research is judged. To get a paper published in a journal, it has to get past 1,2,3 or more referees as well as the editor.
But once it does, hey presto it is “peer reviewed”. Anyone in the business, and probably many outside, know there are problems with the system. As authors, we have all had the experience of being messed around by journal editors and referees. War stories about journals are part of the stock-and-trade of the common room. We all have seen lousy papers published- written by other people of course. Part of the problem is that there are so many journals so with a bit of determination you can get even rubbish published somewhere. That imposes a big demand for referees/reviewers for a tiresome, thankless task (although I think refereeing papers can help one become a better author, to a point). The pressure from funding agencies and universities to produce papers is also a problem.
David Colquhoun, a British pharmacologist, has a nice piece here on this issue particularly from the perspective of the “hard sciences” although it sounds broadly applicable.