Loaders load the values of your config. They can either provide a map of default values (eg. a loader might read a JSON file containing your saved configuration values) or provide a value for each config value individually based on its properties (eg. a loader might check for an env property on each config value type which has the environment variable name containing its value).
See src/loaders for the list of built-in loaders.
You can also build your own loaders.