The Arduino will trigger each transistor individually through a 1Kohm resistor. The collector of each transistor connects to a ground line from the keypad.

9) The Keypad/PIR Sensor Connector – This three pin header allows for you to connect to a resistor based keypad, or a PIR motion sensor module. We will be using these for many of our projects.

As explained by Arduino to Go, each color band on the resistor corresponds to a number but this can differ depending on the band's placement. It's not complicated, but the sheer number of colors ...

The Arduino will trigger each transistor individually through a 1Kohm resistor. The collector of each transistor connects to a ground line from the keypad.