This plugin brings in the ability to write GeoPackage files in GeoServer. Reading GeoPackage files is part of the core functionality of GeoServer, and does not require this extension.
GeoPackage is an SQLite based standard format that is able to hold multiple vector and raster data layers in a single file.
GeoPackage can be used as an output format for WFS GetFeature (creating one vector data layer) as well as WMS GetMap (creating one raster data layer). The GeoServer GeoPackage extension also allows to create a completely custom made GeoPackage with multiple layers, using the GeoPackage process.