Traction roll is when your RC car flips over when turning, even when not making contact with any other cars or barriers. It occurs because the car's momentum "pulls" at its center of gravity, towards the outside of the turn (called centripetal force) while the tires pull the car toward the inside of the turn. If the center of gravity is high enough, this force can flip the car over.
Traction roll, per its name, only happens when the car has enough traction, so it's usually only a concern for smooth asphalt tracks, high-grip carpet or clay off-road tracks.
Check out a traction roll in action: