Peer review is the process of subjecting scientific work to review by other experts in the field. The first, and most notable, step is when a piece of scientific work is looked at by reviewers - peers - for approval prior to publication. After publication, peer review is an ongoing process where a work is open to scrutiny by the scientific community at large, including attempts to replicate the original results independently. The process is designed to ensure that the work meets the standards of the field in question and of science in general.
What peer review is and, quite importantly, what it is not, is key to understanding how science works in practice. The reasons why science uses peer review and its potential problems - bias, mistakes and the limitations of the system - are also important to bear in mind while analyzing how science works in practice.