One of the major pet-peeves of Drupal developers (and developers of other popular CMSes for that matter) has always been the way configuration is handled and deployed from one environment to the next. Drupal 7 stored most of its configuration in the database, so various solutions had to be concocted by developers to get that moved up the ladder as development progressed.
In Drupal 8, great advancements have been made in this respect with the introduction of a centralized configuration system. Although it stores all configuration in the database, it allows it all to be exported into YML files (and then reimported). So, from a development point of view, we have a much better experience if certain features depend on configuration (for example, a new field).
Configuration is also of two kinds—simple and complex (configuration entities we noted in the Entities section). The difference between the two is that the former is always singular. In other words, there is only one instance of itself. For example, the site name and email address are stored inside such a configuration item. You wouldn't expect the need for more than one instance of it. However, in the case of the latter, you would. For example, a View definition is such a configuration entity because it follows a certain schema and we can have multiple View definitions. Makes sense doesn't it?