Full wave rectification requires four diodes.
Working in pairs, the diodes take turns to switch on and off, allowing current to pass through the resistor during both the positive and negative half cycles. all the time. Yet the current passes through the resistor in one direction only!
Notice that we have managed to “flip” (instead of remove) the negative half cycles. Yay.
In practice, a humpy voltage like VR shown above is unusable for most electronic equipment, which require a constant DC voltage. What’s missing is a capacitor to smooth out the humps (with a little bit of ripple). But I’ll just mention it in passing since capacitors are not in the H2 syllabus.
 A capacitor is an electrical component that is able to store charges. Capacitance is not in the H2 syllabus.