Infrastructure as code means your servers, databases, networks and permissions are described in files that are committed to version control, reviewed, and applied by a tool, rather than clicked together in a console by whoever was on call. The declared file is the source of truth, and the real environment is made to match it.
This is what makes an environment reproducible, auditable and recoverable. You can stand up an identical staging environment from the same definitions, see every change to production as a diff with an author, and rebuild after a failure by re-applying the code. "It works because someone configured it once and we hope nobody touches it" is exactly the state it eliminates.
Related terms