This is a simple demonstration of a diode. With a resistor, current and voltage are proportional (by Ohm's Law). With a diode, current and voltage have an exponential relationship.
Current flows from the supply voltage through the diode to ground. The supply voltage can be controlled using the "Voltage" slider to the right. Basically no current will slow if the supply is negative (if the diode is reverse-biased). If the diode is forward-based, very little current flows if the supply is less than 0.6V. As the voltage increases beyond that, the current increases exponentially.