To start, it is important to understand what forward bias and reverse bias means in the electrical circuit sense. Below is an image of a type of diode, a light emitting diode (LED) in specific, and it shows two terminals.
The positive terminal where current enters, denoted Anode, is the longer end. The negative terminal where current exits, denoted Cathode, is the shorter end. In order for this LED to light up, the positive terminal would have to be connected to the positive terminal of a source of sufficient power. Most diodes operate this way. What would happen, though, if the orientation of this was flipped?
Well, current would fail to flow (to a certain point).
Take a look at the simple circuit above. Ordering in the flow of current, there is power, a resistor, then a LED. In this situation, the LED would light up, since its schematic demonstrates forward bias. In the descriptions we used earlier, this means that the LED has its positive terminal hooked up to the positive end of the power source; this is true for this circuit. In short, when one is forward biasing a diode, they intend to allow current to pass given enough power is provided to surpass the voltage threshold of that diode. For the red LED in this circuit, the voltage drop is about 1.8V.
What about this circuit? If I put an ammeter in series with the LED, would it give a reading? Well, yes it would, silly kids, but that reading would be 0. Why? The diode in this situation is reverse biased, meaning the negative terminal is hooked up to the positive terminal of the power supply. It is said that the polarity is incorrect. Since our power supply is DC, or direct current, it will flow in one direction and one direction only. Therefore, current is blocked by this LED. No current flows, and the LED does not light up, no matter the voltage. There is something called the breakdown voltage, though. If you pass enough negative voltage through an LED specifically, it breaks, and current will flow. Although it will not light up, most of the current will pass through. For this red LED, that breakdown is fairly low.
As you can observe, these properties can prove to be very useful when using a different kind of power supply, such as an AC power supply. Alternating Current power switches polarity several times a second. If we switch either of the two circuits above to AC Power at 60Hz (the frequency at which the power supply will switch polarity per second), the LED will very quickly allow current to pass and block current. In reality, the LED will turn on, then off, then on, then off sixty times in one second. You physically cannot see these changes at higher frequencies, but if toned down, or looked through a camera recording at a lower frequency (or even slo-mo!), you can see these alternations.
For voltage rectification, diodes are useful in that they can essentially emulate DC current behavior even if the power supply is AC. On a simple level, if we took one of our above circuits and switched to AC power, the LED would act like a voltage rectifier. When it is forward biased, current will flow and pass through not only the LED but the resistor as well. When the polarity is switched, the LED is now reverse biased and no current will flow. This demonstrates half-wave rectification, meaning the sine-wave of the AC voltage pattern will never switch polarity. It's still not emulating DC, since whenever the polarity switches, there is no current flowing.
So, is there a way to rectify all of the voltage? Is there a way to essentially switch the sine-wave of AC to a line? Yes.
In full-wave rectifying circuits, the diodes are positioned in such a way that no matter the polarity, a section of the circuit will receive a consistent amount of power, similar to DC. Here is a schematic.
Resources
All schematics shown were creating using CircuitLab.
The nice GIF was created in a Circuit Simulator Java Applet.
