The rectifier is a small electronic device that contains four diodes. Diodes are semiconductor devices that allow electricity to flow in only one direction. When AC power is applied to the rectifier, the diodes allow the positive half of the AC waveform to pass through, but they block the negative half of the waveform. This results in a DC output that is pulsating, but it is close enough to DC for the 4-wheeler's electrical components to use.
The rectifier is an important part of the 4-wheeler's electrical system. Without it, the 4-wheeler's electrical components would not be able to function.