Scrub Radius is the moment arm of the arc of steering travel; the distance from the pivot point and the center of the tread at ground level.  Think of it as a door on a hinge;  the further the edge of the door from the hinge, the more it sweeps or scrubs across the floor.  While scrub radius has some benefits on short wheelbase vehicles like racing karts, Indy Cars, Formula 1, Prototypes and most modern road cars run zero scrub radius.  Its essential for a tuner to understand scrub radius.
Suspension Center of Mass Virtual Swing Axle Roll Center Ant-Dive/Squat Bump Steer Scrub Radius Forum