Plain English Meaning
A system that makes sure each generator or alternator on the airplane is doing its fair share of the work, so one isn't overworked while another sits nearly idle.
Full Definition
An electrical control system used in multi-engine airplanes that automatically balances the electrical output of two or more alternators or generators so each one carries an equal share of the total electrical load.
Context Anchor
Seen in aircraft electrical system discussions, especially for airplanes with more than one alternator or generator supplying power.
Derivation
Parallel' here means connected side-by-side to feed the same electrical bus, rather than one after the other. 'Paralleling' is the act of bringing two power sources online together so they share the evenly.