When placed in a simple battery lamp circuit, the diode will either allow or prevent current through the lamp, depending on the polarity of the applied voltage. (Figure below)

When the polarity of the battery is such that electrons are allowed flow through the diode, the diode is said to be forward biased. Conversely, when the battery is “backward” and the diode blocks current, the diode is said to be reversed biased. A diode may be through of as like a switch: “closed” when forward biased and “open” when reversed biased.