The practice of having every change read by another engineer before it reaches the main branch. The reviewer checks correctness, clarity, security, and whether the change fits the existing architecture.
Review catches defects at the cheapest possible moment and spreads knowledge across the team, so no part of the system has exactly one person who understands it.
Every change goes through a pull request with automated checks and at least one human reviewer. We keep reviews small and fast, because a queue of large unreviewed changes is worse than no process at all.