Compile time configuration options allow you to provide different kind of settings based on the environment you're building the Angular app for. You might for instance have a development, staging and production environment. With the Angular CLI you're already perfectly set up. Whenever you compile your app, you just pass the --env=<your-environment>
to the CLI. The according settings will then be compiled into your app. But let's take a closer look how that works and how you can define new environments by yourself