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.