Wheelbase is the horizontal distance between the front and rear axles.
Adjusting the wheelbase changes the weight distribution on the wheels which, in turn, adjusts traction. Adjusting the wheelbase is as simple as moving shims front to back. These shims are located either in at the suspension arm mounts or out near the steering knuckle or rear hub carrier.
Note: not all vehicles have this adjustment.
Making the wheelbase longer increases stability and is better on rough tracks or open tracks with high-speed corners. A longer wheelbase is not recommended for a track with sharp, 180 degree style corners.
Making the wheelbase shorter places more weight over the wheels and helps the car rotate around sharper corners as well as increases steering response.