anti roll bar or sway bar.
best example is think of this , driving your car and you hang a long tight sweeper turn to the left.what happens is due to inertia your car still wants to travel straight even though you are turning left , so what happens is that your vehicle will lean outwards to the right .which if you are going fast enough would cause you to ROLL over

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so since the outer frt and rear wheels are being driven up into the fenders from the vehicle lean what you do is connect the left and right sides of the frt and rear suspension with a BAR .
so now when the outer wheel is driven up this causes the roll bar to move up with it , and since it's connected to the inner wheel as well this pulls the inner wheel up into the wheel well at the same time .this keeps your vehicle level in corners instead of ROLLING OVER.
so the bigger the bar the more leverage can be aplied to the inner wheel to aid in keeping the car level in corners.