Calling C-r with Prefix Argument

[Christopher Dimech]

   Have been reading the output of describe-key for C-r (isearch-backward)
   that does an incremental search backward.
   It states that with a prefix argument, it does a regular expression
   search instead.

   Tried to see the documentation for Prefix Argument but has not helped
   me for the command C-r.

   How does a Prefix Argument work with C-r and what happens?

   Regards
   C*