A diode is a device that only allows current to flow in one direction. That is why it is used in rectification i.e. changing AC to DC.
When a diode is forward biased it will allow current to pass through, as in 4 A
However, when a diode is reverse biased it will not allow current to pass through, as in 4 D
to completely understand how diodes work would go past the A level syllabus as you would need to understand P and N type semi conductor doping and then valence and conduction bands. With that, you will be able to explain why the diode conducts in one direction.
But for A level just know that if the anode of a diode is connected to the positive terminal of a power supply it is considered Forward Biased and will allow conduction.