gnus: What is nnir?

 
 8.1.1 What is nnir?
 -------------------
 
 ‘nnir’ is a Gnus interface to a number of tools for searching through
 mail and news repositories.  Different backends (like ‘nnimap’ and
 ‘nntp’) work with different tools (called “engines” in ‘nnir’ lingo),
 but all use the same basic search interface.
 
    The ‘nnimap’ and ‘gmane’ search engines should work with no
 configuration.  Other engines require a local index that needs to be
 created and maintained outside of Gnus.