A serial port may be added to the Mac via a USB-to-serial converter (like the Sabrent USB to serial cable that uses the Prolific PL-2303 chip - drivers)
To talk to it, you need a terminal emulator. This can be with the built-in Terminal application. (I found out how here.)
Find available tty devices:
ls /dev/tty.*
Pick the one you want (e.g. /dev/tty.usbserial) then use by
screen /dev/tty.usbserial 9600
Quit by typing
control-A
control-\
Some terminal emulators are: