Definition:
Reviews or Inspections are the validation techniques in which one or more artifacts are examined critically by a group of qualified people. The goal is to determine whether the artifacts fulfills the demands of its target audience with sufficient quality, easy to develop, maintain and enhance. The defects or problems are reported to the creator of the artifact so that they may be potentially addressed and improve the quality.
Another goal of reviews is to transfer the skills and knowledge to other team member, so as to cultivate a learning attribute in each team member. This gives an opportunity to discuss what is good and not-so-good about the artifact creation, why and how to approach the same sort of mistake in the next artifact creation. Through this activity people can quickly learn from one another.
Further Reading:
Book: Disciplined Agile Delivery by Scott W. Ambler and Mark Lines