The Nova Force is a wellspring of energy produced and stored by the Xandarian Worldmind. It is a fractional expression of the energies of the Power Cosmic and the signature capability and offensive tool of the Nova Corps.
The distribution method is derived from observations of the Green Lantern Corps. The Worldmind adjudicates the power levels largely on a basis of rank and personal capability, limiting the flow-through of energy to unproven Nova Corps and permitting more trusted ones to access more power at faster rates. The Nova Force is primarily used as an offensive weapon, directing blasts of raw high-energy particles or manipulating gravitational patterns in reality.
There are finite limits to the amount of power an individual Nova Corps soldier can channel safely. Training, experience, and clear mental focus contribute to increasing this amount. There are soft limits for the Worldmind too, though most of the galaxy would estimate these limitations to be theoretical.
The source of this wellspring of energy is the collective consciousness of the brains that compromise the gestalt of the Worldmind.