Short answer: peer review is controversial because it is useful but imperfect. It can catch weak reasoning, poor methods, and unsupported claims, but it can also be slow, inconsistent, biased, or unable to detect fraud.
Peer review usually means that a manuscript is assessed by other researchers before publication. Reviewers may ask whether the question matters, whether the methods fit the question, whether the analysis is sound, and whether the conclusions go beyond the evidence. In the best cases, this improves the paper before readers ever see it.
Why people defend it
Peer review gives academic publishing a quality filter. It does not mean a paper is true, but it means the paper has passed at least one stage of expert scrutiny. That matters because most readers do not have the time or specialist knowledge to inspect every method, data table, and statistical choice for themselves.
Good peer review can also protect the literature from obvious errors. Reviewers may spot missing controls, weak citations, unsupported claims, unclear language, or interpretations that are too ambitious. A strong review process should make research more careful and more useful.
Why people criticise it
The controversy comes from the gap between what peer review is and what people imagine it to be. It is not a fraud detector. It is not a guarantee of reproducibility. It is not a democratic vote of all experts in a field. Often, two or three busy specialists are asked to evaluate a paper without payment, under time pressure, and sometimes without access to all underlying data.
Peer review can also be conservative. Unusual ideas may face resistance. Famous authors may receive more benefit of the doubt. Reviewers may disagree sharply. Journals may struggle to find qualified reviewers. In some cases, peer review has been manipulated through fake reviewer identities or paper-mill activity.
The right conclusion is not that peer review is useless. It is that peer review should be treated as one checkpoint, not the final word. A peer-reviewed paper deserves attention, but it still needs replication, transparency, criticism, and time.