A second pair of eyes on a change
Before a checkout feature joins the shared code, another developer reads the change, asks about unusual cases, and suggests improvements. This is code review. The reviewer may also look at the ticket, design, and test results.
Reviews often happen in a pull request, where people can comment on specific lines and approve the change. They can also happen while developers work together or discuss code in person. A pull request is the proposal; code review is the examination.
Review is a conversation, not a guarantee
Useful feedback explains the concern and suggests a way forward. Reviewers may check behaviour, readability, security, tests, and how the change fits the system. Automated tests catch some problems but cannot assess every design choice. Approval does not guarantee bug-free code or mean the change is live for customers.