VOCs are the organic solvents that leave a coating as it dries. They are regulated because they contribute to ground-level ozone and photochemical smog, and because of workplace exposure.
VOC content is usually expressed in grams per litre of coating, though the definition of what counts as VOC and how it is calculated varies between jurisdictions — which matters when you are declaring compliance across markets.
Three routes reduce it. Water-based systems replace most organic solvent with water, though coalescing solvents still contribute. UV-curable systems are essentially VOC-free, since reactive diluents polymerise into the film rather than evaporating. High-solids systems reduce the solvent fraction without changing platform.
A point worth noting: VOC compliance is assessed on the complete formulation as applied, not on the resin. A low-VOC binder in a high-solvent formulation is not a low-VOC coating.