For LEDS the so-called "forward bias" voltage will indicate.
The forward bias voltages is that voltage, which an LED begins to shine and which forms at the LED during radiation. If the voltage at the LED sinks under the forward bias voltages, then no more current flows and... darkly it stays. If the operating voltage is higher and the correct pre-resistor is available then the LED regulates itself on the forward bias voltages and the remaining voltage de-energize at the resistance.

What is RESISTANCE?
The less free electrons in certain materials, which is present resistance, e.g. the carbon(carbonlayerresittance), are available the less it for a current flow in the electric circuit. If e.g. a broad, slow river is forced to flow by a short tightness then it becomes rapid on this short distance and forms a very fast, strong river.
Thus it behaves also in an electric circuit. The resistance R (a tightness), let only few free electrons per time throu and gives thereby to the entire electric circuit the size of the current flow (serial connection). The free electrons can flow in leaders nearly unhampered, but in the resistance they must "rub" itself at the bound electrons. So strongly that the resistance becomes sometimes warm. The bulb e.g. shines, because the thin wire(Wolfram) begins to glow under the influence flowing through the current (friction). Therefore the bulb is so uneconomic and produces much more heat than light.

With a resistance the resistance material is applied on a ceramic stick and covered with a protective layer. The inscription takes place in the form of color rings. Apart from the resistance value also the accuracy class is indicated.

The color code: