An object's acceleration is constant and downward due to the force of gravity In projectile motion, but the object's velocity reverses direction during the object's motion. This is because of the inertia described in Newton's first law of motion and because acceleration and velocity are two different physical quantities. Velocity is the speed and direction of an object, while acceleration is the rate that the velocity changes.
By Newton's first law, an object in motion tends to stay in motion unless acted upon by a force. This force causes an acceleration in the object. When the acceleration is in the same direction as the velocity, the speed of the object increases, and when the acceleration and velocity are in opposite directions, the speed decreases.
In projectile motion, the force of gravity is constant and downward, and the object is moving upward against the force of gravity, which slows it down. At the object's peak of motion, its speed reaches zero, and thereafter, the object moves downward toward the ground. At this point, the object's velocity is downward, while its acceleration is still constant and downward. The object then speeds up, because its velocity and acceleration are in the same direction.