Какова функция wget -O -?

Я использовал **, чтобы указать, что это важно. из man wget

-O file
   --output-document=file
       The documents will not be written to the appropriate files, but all
       will be concatenated together and written to file.  ** If - is used as
       file, documents will be printed to standard output, disabling link
       conversion.  (Use ./- to print to a file literally named -.) **

       Use of -O is not intended to mean simply "use the name file instead
       of the one in the URL;" rather, it is analogous to shell
       redirection: wget -O file http://foo is intended to work like wget
       -O - http://foo > file; file will be truncated immediately, and all
       downloaded content will be written there.

и из man apt-key

add filename
       Add a new key to the list of trusted keys. The key is read from
       filename, or ** standard input if filename is -.**

это объясняет вашу команду.