Why An LED is a diode, so current only flows when the anode is positive relative to the cathode, which is forward bias. Electrons and holes recombine across the junction and release energy as photons, so light output is proportional to forward current. Typical LEDs drop roughly 1.5 to 3 volts depending on color, so a series resistor is used to set the current, commonly 10 to 20 mA.
Watch out Reverse bias blocks current in an ordinary diode and produces no light; that condition belongs to Zener diodes, which are deliberately run in reverse breakdown as voltage references, not as light sources.
LED = Light Emitting Diode: light only when current flows forward, just like any conducting diode.