Arduino spectrum analyzer shield schematic

Using Arduino Audio
3

From Details in
http://arduino.cc/forum/index.php/topic,8328.0.html

  • A 3.5mm Audio in connector for input.
  • Another 3.5mm connector for output to your speakers/stereo.
  • 7 Band real time spectrum analyzer (Stereo!) 63Hz, 160Hz, 400Hz, 1KHz, 2.5KHz, 6.25KHz, 16KHz
  • Only uses 2 digital pins and one analog.
    But see the schematic I can't understand that one audio jack is input and one is output. and I see this schematic use two analog.
    Can you help me to understand?