This is a serial interface kit that allows the Z80 computer to communicate with a PC running terminal emulation software or with a dumb terminal if you have one. The interface runs at 9600 baud. To be useful, you should have a version 7 or higher EPROM to take full advantage of the new code that is there, including a monitor program. However, you can use the serial interface with EPROMs version 6 and lower, by entering a 30-byte boot loader program through the port switches.
An optional EPROM with Palo Alto Tiny BASIC is available for use with the computer with the serial interface attached. The Tiny BASIC manual is here.
Note there was a typo on page 13 of the serial interface instructions downloaded prior to June 29, 2015. ROM entry should be 0x04C0, and NOT 0x0C0.