Plain English Meaning
A built-in system that makes sure two power sources working at the same time share the workload evenly, instead of one doing most of the work while the other coasts.
Full Definition
Electrical circuitry in a multi-engine airplane that automatically balances the electrical output of two or more alternators or generators so each unit carries an approximately equal share of the total electrical load on the airplane's bus.
Context Anchor
You encounter this term in discussions of multiengine aircraft electrical systems, especially when more than one alternator or generator can power the same equipment.
Derivation
"Paralleling" comes from the electrical term parallel, meaning two power sources connected so they feed the same circuit at the same time. The circuitry's job is to keep those parallel sources working together cleanly.