Build the circuit shown using a red LED. The resistor R1 prevents an excessive current flowing through the LED.
The LED is now connected in the forward direction and lights up.
In a next step turn the LED around. It is now in the reverse direction and does not light up...
The LED shows the behavior of a normal diode. It only lets current pass in one direction.
In the next experiment you examine the voltage required by the LED to light up...