GaN chargers are more power-efficient chargers in which their main power-conversion component, the Field Effect Transistor, is based on Gallium Nitride (GaN) instead of silicon. That allows the charger to reduce power loss during conversion, and GaN chargers also support higher voltage at lower heat levels.
These advantages allow GaN chargers to be noticeably more compact (higher power density) at a given Wattage, which any consumer can benefit from. Nobody wants to carry a huge and heavy power adapter, especially for a relatively compact laptop or phone.
Finally, GaN technology can also reduce the cost of the charger for a given Wattage. Because of all these benefits, the industry is moving away from Silicon to GaN for these charging devices.
Gan also benefits other industries such as renewable energy, data centers, and wherever power conversion is needed. That said, most consumers will read about GaN in the context of GaN chargers.