A ball is thrown vertically upward its velocity at the highest point is zero.
When a ball is thrown vertically upward, gravity (gravitational force of Earth) constantly pulls it downward, opposite to the direction of motion.
This means the vertically upward-moving ball actually has a retardation of motion during its upward movement.
As a result, its velocity falls as it moves upwards following the motion formula v= u – gt.
At the highest point of its motion, the velocity of the ball becomes zero.
And just after this its downward fall starts with acceleration = g.