Initially when ball is thrown up in air, its kinetic energy goes on decreasing. This loss in kinetic energy changes in potential energy and it keeps on increasing. This potential energy becomes maximum at highest point of its motion. At this highest point, kinetic energy of ball becomes zero. Now while coming down, potential energy goes on decreasing and this loss changes into kinetic energy. This kinetic energy keeps on increasing till it hits the ground or stopped by some external agent. Hope it helps.