Given that a roller coaster changes its direction and speed over time.
Velocity is a vector quantity. As all the vectors the velocity has magnitude, i.e., speed and direction.
The acceleration is described as the time rate of change of velocity. That is when the velocity changes there will be an acceleration. And as the velocity is the vector, the change in velocity may mean the change in the speed or change in the direction of change in both speed and the direction.
Thus the acceleration occuring when the roller coaster is moving corresponds to both the change in the magnitude and direction of the velocity of the roller coaster.