The "TRAC off" alert light found on some vehicles is a visual aid that indicates the vehicle's traction control feature is not currently engaged. Traction control systems are a part of a vehicle's ant-lock brake system and are intended to prevent the loss of traction between the wheels and the roadway.
Traction control systems originated in 1971 when Buick introduced a new feature called MaxTrac. The system used a computer to detect wheel spin and provide more power to those wheels to counteract the spin. While a loss of traction can be caused by many things, it is most often a result of wet roadways and loose gravel.
The system, commonly referred to as TCS, can also work closely with the anti-lock braking system by applying friction to wheels that are spinning faster than the others. Vehicles detect this via sensors in each wheel that sense speed changes. This applied friction causes the wheel to slow down and provides an opportunity for the wheel to once again grab the roadway.
Not all vehicles contain a TCS, but in those that do, the function can generally be turned on and off as needed. To determine whether a specific vehicle has a traction control system, owners would need to consult their owner's manual.