When we throw an object in the upward direction, we provide it with certain initial velocity due to which it covers a certain distance up to the maximum height.
While the object is moving in the upward direction, its velocity keeps on reducing due to the acceleration due to gravity which acts vertically downwards in the opposite direction thus reducing its velocity.
So, the maximum height attained by the object is the point where this upward velocity of the body becomes zero and after that the object starts to fall down.
