Version control
Also known as: Git
Version control records every change to a codebase — what changed, when, by whom, and why. Git is the system essentially everyone uses. It lets developers work in parallel on separate branches and merge their work, and it lets any past state be recovered exactly.
Beyond coordination, it is the project's memory. The reason a peculiar-looking line exists is usually recoverable from the commit that introduced it, which is what makes a codebase maintainable by people who did not write it.
It is also the foundation everything else stands on: CI/CD is triggered by it, code review happens through it, infrastructure as code lives in it, and a rollback is a matter of deploying a previous commit. A project without it is not a project with a minor gap; it is a project with no way to answer what changed.