The diagram shows the force needed to move a 1kg mass in a gravitational field (N) with the distance moved (m). Work done = force × distance moved in the direction of the force. The area under the graph is the work done on the 1kg mass and is transferred to gravitational potential energy. Work done per unit mass = gravitational potential, so, provided the mass is 1kg, the area under the graph from a point out to infinity is equal to the size of the gravitational potential at that point. The potential itself is negative, because work has to be done on the mass to move it out to infinity where the potential is zero.