Jimu is the name of the framework for ArcGIS Experience Builder and it contains the following packages:
jimu-corepackage 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-for-builderpackage provides classes for developing the widget setting page such as
jimu-for-testpackage contains classes, types, and functions that can be used for unit testing.
jimu-uipackage includes UI components to be used in your experiences.