tradotto a parole

find /tua_dir : trova nella directory /tua_dir

-name *.mp3 : tutti i file che si terminano .mp3

- exec conversione.sh : ed esegui conversione.sh

{} : su ciò che hai trovato

\; : sintassi per terminare il comando lanciato da exec

Dal man di find


-exec command ;
Execute command; true if 0 status is returned. All following arguments to find are taken to be arguments to the command until an argument consisting of `;' is encountered. The string `{}' is replaced by the current file name being processed everywhere it occurs in the arguments to the command , not just in arguments where it is alone, as in some versions of find. Both of these constructions might need to be escaped (with a `\') or quoted to protect them from expansion by the shell. The command is executed in the starting directory.
In poche parole è come se fosse un nome di una variabile.