Retardation is negative acceleration or deceleration. The velocity of a body may increase or decrease. Positive acceleration increases the speed of a moving object so the object moves faster and faster. Negative acceleration or deceleration has the opposite effect and it reduces the speed of a moving object. Retardation is the rate of decrease in velocity.
If the driver pressed on the brake instead of accelerator then the motion of the motor-cycle becomes slower. Its velocities may decrease to 5 m/s per second. The rate of decrease in velocity of the motor-cycle per unit time is negative acceleration or retardation.