Config Split is a contributed module that lets you manage different configuration sets for different environments. For example, you might want development modules enabled locally but not on production.
It works by splitting your configuration into a default set plus one or more environment specific sets. When you export config, each split writes to its own directory. On import, only the active splits for the current environment are applied.
Common uses include enabling Devel on local only, using different mail servers per environment, and keeping certain settings out of version control. Config Split has become a near essential part of professional Drupal workflows when managing multi environment deployments. It solves a real problem cleanly and integrates well with standard Drush commands for exporting and importing configuration.