The purpose of this widget is to give you an overview of how many of the keyword searches performed result in zero hits at a given time interval, and a top list of these queries.
A zero hit happens when the user searches for something, and the search returns zero documents in the search result list.
Note that zero-hit statistics are not applicable to semantic search, as it always returns results. Even when no highly relevant matches are available, the system provides the closest possible results to ensure a seamless search experience.
This widget has the exact same setup as the Searches widget, so for more details see the documentation of that widget here.