Zero balance is a term used to describe when all the attached items added to the crank such as flywheel, vibration dampner, clutch etc do not have any imbalance (attached weights)within them to offset an imbalance in the spinning crank, rods and pistons, pins and rings. The crank counter weights have enough mass to offset the weight of the rods, pistons, bearings, rings, and clinging
oil. THEN any "0" balanced flywheel, or damper can be attached (or changed) without the necessity of rebalancing the entire assembly.
There is less stress on the main bearings if it is "0" balanced and it is generally smother running.
An externally balanced engine such as a 289/302 can be converted to internal balance by the addition of slugs of Mallory metal (heavier by volume than iron) pressed and welded into holes drilled into the crank counter weights.