Simple Explanation here...
The purpose of that board is to convert voltage levels.
Standard logic levels for ICs and small devices is 0-5v, like the RC or OI.
However, when transmitting data over a wire, it is nice to have more noise margin,
and one way to achieve this is to jack the voltage differential. RS232 voltage levels
do just that.
Your standard legacy pc has a serial port that uses RS232 voltage levels, whereas
the RC has a TTL level serial port. I built one of those modules last spring
to interface the edubot to a matrix orbital lcd screen.
The link below has more detailed information.
http://www.beyondlogic.org/serial/serial.htm#1
I assume, w/o having access to a kit right now... that the rs232->ttl converter
is just a max233cpp chip or equivalent with appropriate connectors.
The digikey part number for that chip is: MAX233CPP-ND
The datasheet is:
http://rocky.digikey.com/WebLib/Maxi...u%20MAX249.pdf