The INSPIRE extension allows GeoServer to be compliant with the View Service specification put forth by the Infrastructure for Spatial Information in the European Community (INSPIRE) directive.
In a practical sense, the INSPIRE plugin extends the WMS capabilities document to include the following extra information: Metadata URL, or the link to the metadata associated with the WMS layers; and SupportedLanguages, for detailing the default language.
The current INSPIRE extension fulfills “Scenario 1” of the View Service extended metadata requirements. “Scenario 2” is not currently supported in GeoServer, but is certainly possible to implement. If you are interested in implementing or funding this, please raise the issue on the GeoServer mailing list.
For more information on the INSPIRE directive, please see the European Commission’s INSPIRE website.