The ‘Speed’ box allows you to choose the speed (or ‘baud rate’) at which to talk to the serial line. Typical values might be 9600, 19200, 38400 or 57600. Which one you need will depend on the device at the other end of the serial cable; consult the manual for that device if you are in doubt.
This configuration setting is also visible on the Session panel, where it replaces the ‘Port’ box (see section 4.1.1) if the connection type is set to ‘Serial’.