Negative acceleration is nothing but retardation.
Acceleration is rate of change of velocity.
But negative acceleration means that the rate of change of velocity is negative or velocity decreases.
Example:
(1) When we apply brakes in a moving car, then negative acceleration acts on it and the car stops.
(2) When we throw a ball upwards, then also negative acceleration acts on it. Thus, when the ball reaches the highest point, velocity of the ball becomes zero.
