The following explanation is guesswork. This message is shown when viewing the credits of a page (example: https://translatewiki.net/w/i.php?title=Main_Page&action=credits). Note that this action is disabled by default (currently enabled on translatewiki.net - to use type &action=credits at the end of any URL in the address bar).
The message appears at the end of the list of credits given in the message("Based on work by $1.") if the number of contributors is above a certain level.