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
:-) Dave