Connecting RFID reader with RS232 interface to an arduino
Your Arduino's microcontroller has a UART (Universal Asyncronious Receiver/Transmitter) which can be programmed to be connected to a pair of pins on the board. This uses the same signals as your RFID reader, except at different voltage levels (and inversion). To connect the two, you need a converter, as described in the link you provided.

If you were to directly connect the RFID reader to your Arduino, you would probably damage the Arduino due to the voltage differences.

If you don't want to breadboard the circuit, you could use something like http://www.cutedigi.com/arduino-shields/rs232-shield-for-arduino.html