The question comes up quite often so I thought I'd just take the time to write it up here in the how-to forum for a reference.
Polarization restores the residual magnetism in the cores of the generator armature so that when the generator starts charging it will do so in the proper polarity.
It is usually not necessary to re-polarize unless the generator has been removed and repaired or the regulator has been changed. If this has occured, then re-polarization must be done before starting the tractor. Otherwise, the generator may not charge or it could charge in the reverse direction.
Re-polarizing is relatively easy to do. All that is necessary is to briefly connect the generator armature to the battery hot terminal. The most convenient way to do this is to take a short jumper wire and touch it to the "Batt" and "Gen" terminals on the voltage regulator. At the moment of contact you should see a slight spark. This indicates that the process is completed.
It should be noted that it is the generator that is being polarized, not the regulator.