In Drupal, when you step into configuration, you are entering the realm of a site-builder.
A site-builder is concerned with pretty much everything in a Drupal site - from the way a site's theme looks, to how the information architecture is organized, to how different pages are laid out, to how content and users are managed.
Everything within a Drupal site is configured, to some degree or another. Configuration is what makes your site behave the way it does and it's what allows you, and your users, to interact with your site efficiently and safely.
The road to becoming an expert Drupal site-builder branches into many different areas, but, starting out, a solid understanding of entities: what they are, why they're used, and how to work with them, is really essential.