This allows applications/services (e.g. Akonadi, Sink, KHolidays etc) to provide calendar content to other applications/services (e.g. KOrganizer, Plasma, Calindori). The plugins expose a set of CalendarEntries, which are a thin wrapper around a KCalendarCore::Calendar and some metadata.
This is intended to replace the calendar plugin system found in https://cgit.kde.org/kdeclarative.git/tree/src/calendarevents