The vertical velocity of an object is the speed at which it is moving up or down. At the highest point of a projectile's trajectory, the vertical velocity is zero because the object is not moving up or down at that moment.
To understand why this is, it's helpful to understand the concept of projectile motion. Projectile motion is the motion of an object that is projected into the air and then is subject to the force of gravity. When an object is projected into the air, it initially has both a horizontal velocity (the speed at which it is moving left or right) and a vertical velocity (the speed at which it is moving up or down). As the object travels through the air, the force of gravity acts on it, causing it to accelerate downward. This acceleration results in a decrease in the vertical velocity of the object as it rises, and an increase in the vertical velocity as it falls.
At the highest point of the projectile's trajectory, the vertical velocity is zero because the object is momentarily at rest with respect to its upward or downward motion. At this point, the object is moving horizontally at a constant speed, but it is not moving vertically. As the object begins to fall back down, the vertical velocity will become negative, indicating that it is moving downward.