Code review
Teammates reading and critiquing a code change before the project accepts it.
What it tells you about a developer
Teams route reviews to the people they trust to judge code. Heavy review activity in a serious project marks a gatekeeper, and usually senior standing. Thoughtful review comments are direct evidence of written communication skill.
Code review is the main quality gate on professional teams. A proposed change typically needs at least one approval before it lands. Review comments on open source projects are public, so you can read exactly how a developer gives criticism and how they take it.