Sitecore 9 introduced a new concept called Configuration Layers. These layers are dedicated folders in the App_Config folder to provide clear separation between Sitecore and custom configuration files. Sitecore 9 has also introduced the concept of rule-based configuration which allows you to more easily configure your individual instance roles when you start scaling out your infrastructure.
To get an introduction to the new configuration changes in Sitecore 9, watch my video on Configuration Layers and Rule-based Configuration (5:15)
You can learn more about the changes to the configuration layers by visiting the official documentation site: