The most common LEDs are 5mm diameter. They glow (usually red) when current flows through them (current can only flow if the anode is more positive than the cathode). The LED has a forward voltage of 2V, i.e 2 Volts is dropped across the LED when it is in operation. It requires a maximum current of 30mA. The resistor in the circuit is included to limit the current to around this value.