An Orbital Defense Generator, or ODG, is a large groundside power plant used to provide the energy needed by Orbital Defense Platforms to fire their primary weapon.
Prior to the First Battle of Reach, Reach had seven ODGs to power twenty ODPs. As demonstrated during the battle, ODGs were vulnerable to ground assault, and the failure of UNSC ground forces to protect the power plants and the resulting nullification of the planet's ODPs was a major factor in the battle's outcome and resulted in the loss of Reach.
Speculation
Although never explicitly elaborated upon, it likely seems that the means of power transmission from the ODGs on the ground to the ODPs in orbit was through a means similar to rectennas, a form of a high powered antenna.