What does that light mean??

ABS is essentially an add-on to the existing brake system.  It only comes into play when traction conditions are marginal or during sudden panic stops.  The rest of the time, it has no effect on normal driving or braking.

When the ABS warning light comes on this indicates that the ABS system has been deactivated because of a self-diagnosed fault.  Normal braking should remain; the vehicle should be safe to drive provided the driver does not have to call on his ABS system when panic braking on a wet or slick surface. 
 
An ABS warning light may also indicate a loss of power-assist if the vehicle has an ABS system relying on an electric pump and pressure accumulator pump rather than a conventional vacuum booster.
 
The car will still brake, but will not have the usual power assist.  This could create an unsafe situation for drivers who have difficulty braking without the power assist.
 
An anti-lock warning lamp that comes on when the car starts moving, or anti-lock braking operation or valve cycling that occurs during normal stops on dry pavement often indicates a problem with one of the wheel speed sensors.
 
When both warning lamps are on and power assist is present, it may indicate a low brake fluid or loss of hydraulic pressure in one of the brake circuits.
 
If brakes are pulling or rubbing during normal braking, it is not an ABS problem.  The vehicle has a conventional brake problem needing attention.
 
A brake warning lamp (not ABS lamp) that remains on or comes on while driving usually signals a problem with the hydraulic system, not the ABS system.  There may be a fluid leak or loss of pressure, either of which pose a danger to safe braking.  The cause of the brake warning light should be investigated immediately.