The difference between a module’s install (i.e. required) config and optional config directories doesn’t seem well documented in Drupal 8, so this answer was quite informative:
install: The folder can contain configs, like a view or any other config. All configs will be installed, if any config fails module can’t be installed.optional: The folder can contain configs, like a view or any other config. All configs will be installed if possible. If config has missing dependencies, it won’t be installed.