Peer-review is the system used to assess the quality of a manuscript before it is published. Independent researchers in the relevant research area assess submitted manuscripts for originality, validity and significance to help editors determine whether the manuscript should be published in their journal. You can read more about the peer-review process here.
The benefit of double-blind peer review is that it allows reviewers to judge the manuscript based on content alone, and they are not unconsciously biased by knowledge of who the authors are.