Jimu is the name of the framework for Experience Builder and it contains the following packages:
jimu-core: provides the interfaces and types to work with widget props, message actions, data source, etc. It also defines several classes that will be used throughout your experiences.
jimu-ui: includes UI components to be used in your experiences.
jimu-for-builder: provides classes for developing the widget setting page such as SettingChangeFunction and WidgetSettingProps.