Plugins are a way to extend the capabilities of the World Builder. They are inside a dll file in the Plugins folder of the World Builder.
Depending of the type of the plugin, the plugin behaves differently. The most common ones you’ll be using are Wizards, Tools, AssetsProviders and Localizations. Wizards are for a step-by-step guide for the user through some setup or content creations. Tool plugins are for extending the World Builder in general. AssetsProviders are for importing additional assets from other sources, e.g. your asset database. Localization plugins add localizations for your plugin, so that UI elements can be translated in other languages.