When a gravitational force acts on a body and causes it to
move through some distance, the gravitational energy is the
gravitational force multiplied by the distance through which
the body moved. For example, when we drop our hand to
our lap or to our side, it loses gravitational energy. Where
did this energy go? Well, before our hand hit our lap or
jerked to a stop by our side, it was moving, right? Just
before our hand stopped moving, almost all of the
gravitational energy that it had while we held it out in front
of us was converted to kinetic energy (energy of motion).
And what about after our hand stopped moving? Where did
its kinetic energy go then? To heat. (our hand and our leg
where our hand hit it are slightly hotter than they were
before.)