The voltage is the electrical force that makes the electricity move through a wire and a circuit. Think about it as the pressure in a water pipe. You can measure it in terms of volts.
Let's measure and experiment
On your Arduino shield, put together a small circuit by connecting a resistor (330 ohms) to the VS (the +5V Supply) and an LED connected to the resistor and the ground (GND). Plug in your Arduino board and the LED should light up.
If the LED does not light up, check your resistor value (orange, orange, brown) and make sure the LED is in the correct orientation (long lead with the resistor and short lead to the ground).
Use your Voltmeter as indicated in the magazine and measure the voltage across the LED. This will be the value of VL. To know the value of VR, measure the value of VS by using the thick red wire and clip and attaching it to VS (+5V Supply).
VR = VS - VL (The voltage supply minus the voltage across the LED will give you the voltage across the resistor).