The ASR Button is assisted traction control. It assists in the traction
distribution of power to your wheels under hard acceleration, it also
reduces the power to a wheel spinning on ice. or if you are cornering to
fast and a wheel starts to spin it will reduce the power to that wheel by
applying the brake on that side.
The ASR is always on and only when the button is pressed does it turn it
off and a symbol is displayed in the dash.
HOWEVER if the symbol is on in the dash always then there is a chance that
your ABS is faulty