Many modern automatic gearboxes are controlled by software inside a transmission control unit, commonly called the TCU.
This software helps manage gear selection, shift timing, torque-related behaviour and other functions according to the gearbox design.
Gearbox remapping changes supported software settings to alter how a compatible transmission behaves.
On certain vehicles, this can improve shift response or change the points at which the gearbox selects a different gear.
Some transmissions also support additional software options, although these vary by gearbox type, software version and vehicle specification.
Gearbox tuning is separate from an engine ECU remap. It doesn't automatically increase the engine's horsepower, and it isn't a repair for mechanical gearbox faults.
The correct approach depends on the exact transmission fitted to your vehicle.