Question

What does acceleration in and against the direction of motion mean?

Answer 1

When acceleration is in the direction of motion the magnitude of velocity increases means that the speed is the body is increasing.
eg- An example of acceleration in the direction of motion is a smooth spherical stone dropped from the roof of a building on a windless day. All the motion is straight down, toward the center of the earth, and so is the acceleration.

When acceleration is opposite to (against) the direction of motion, the magnitude of the velocity decreases, meaning that the object slows down. An example is the application of the brakes to slow or stop a car. During the time that the pressure remains on the brake pedal, the car's acceleration is negative ... meaning opposite to the direction of motion ... and the car's speed is decreasing.