Rector is a tool that allows developers to instantly upgrade and refactor legacy code by processing source code directories and migrating them to new frameworks, standards or any other changes with one simple command. The document discusses how Rector was used to migrate a Nette project consisting of over 54,000 lines of PHP code to Symfony in just one month by running a single command. It also explains how Rector can be used to fix PHPStan errors, remove dead code, and boost code quality in CI/CD pipelines with additional commands.