Retardation is nothing but a negative acceleration. The velocity of the body may either increase or decrease. The change in velocity is known as acceleration. If the velocity of the body increases, acceleration is said to be positive. Similarly, if the velocity decreases, the acceleration is said to be negative.
The train reaching the station slows down, in this case, we can say that the train is retarding. Retardation is acceleration with a negative sign. Or the negative value of acceleration shows that the velocity of a body is decreasing.