It's Toyota's Traction Control.
When active, it prevents the wheels from spinning by reducing torque if it detects a wheel is losing traction. It also reduces braking force if it feels the tyre is slipping on the road surface. Perfectly safe to leave active all the time.
Edit: Perhaps I should add that it's ON when the light is OFF. The light is a wanring light telling you it's been deselected, so make sure he drives with no light on the switch.
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