It depends on what you want to do. Do you want to run a shell or applicaiton interactively from the terminal, connect out to another computer over the serial line, automate communication with a device over a serial port?
If you want bidirectional communication then I presume you want something interactive with a human on the terminal. You can configure the system to allow logins from a terminal over a serial port by seting up a getty(1) session on the serial port - getty is the tool for setting up a terminal and allowing logins onto it. Put an entry in your inittab(5) file to run it on the appropriate serial port on a
If you want to connect to a device and initiate automated two way conversations then you could see if expect will get you what you want. Use stty(1) to configure the port to the right parity, baud rate and other relevant settings.
If you want to communicate interactively with another computer over the serial port then you will need terminal emulation software. This does quite a lot - it sets up the port, interprets ANSI or other terminal command sequences (ANSI was far from being the only standard supported by serial terminals). Many terminal emulators also support file transfer protocols such as kermit or zmodem.
The ins and outs of serial communications and terminal I/O are fairly complex; you can read more than you ever wanted to know on the subject in the serial howto.