Sorry: military doctrinal term. Very roughly, the center of mass is that which allows one force to influence another. The center of gravity is that which, when removed, prevents the exercise of power. The idea behind the modern maneuver doctrine is to identify and destroy your opponents center of gravity, vs. his center of mass. E.g. Who cares if the Soviets have 20,000 tanks if they can't get fuel for them; 20,000 metal pillboxes aren't nearly so dangerous.
For example, in the former Iraqi regime under Saddam Hussein, the center of mass was the Iraqi military; the center of gravity was the government. Al-Qaeda's center of mass is probably just its personnel. Al-Qaeda's center of gravity, on the other hand, is far more difficult to determine. It's not Al-Qaeda's leadership alone--the West has destroyed much of that, yet the organization remains dangerous. One might argue that it has multiple centers of gravity, each of which must be destroyed/contained/weakened: madrassas promoting its idealogy; implicit support throughout much of the Islamic world; etc.
In Elemental, one center of gravity is almost certainly the Channeler: if s/he goes down, game over. Whether or not the Channeler is a center of mass is likely a play-choice: do I have an amped up Channeler who eats armies for breakfast, or do I spread out the Essence into other things? For some factions, a center of gravity might be the economy, while the center of mass is the Bear Cavalry.
Does this explain the concept well enough?