The 555 timer is an IC that can be used for a variety of timer and multi-vibrator applications. In this project, the 555 timer is used to create a touch switch (circuit based on example by Forrest Mims).
Use a 100kΩ resistor, a 0.01 µF Capacitor and a 10 µF Capacitor. The yellow wire serves as the touch sensor, and when touched, pin 3 on the 555 timer will output HIGH to Arduino's input pin 6.
check out Forrest M Mims' «Engineer’s Mini-Notebook - 555 Timer IC Circuits» for more information.