How a breadboard is internally connected:

For a Light Emitting Diode to emit any form of light it needs a current to flow through it, as it is a current dependant device. As the LED is to be connected in a forward bias condition across a power supply it should be current limited using a series resistor to protect it from excessive current flow. From the graph above we can see that each LED has its own forward voltage drop across the PN-junction and this parameter, which is determined by the semiconductor material used, is the forward voltage drop for a given amount of forward conduction current.