Lunar Orbit Rendezvous explained in terms of gravity well efficiency. If on an expedition to the bottom of a canyon, it is a waste of energy to transport everything needed, including what is only needed for the return trip home. It is much more efficient to leave the return supplies at the top of the canyon, and get them on your way back out after having reached the goal at the bottom of the canyon bringing only the supplies necessary for that part of the expedition. Likewise, when descending the gravity well of the Moon to reach the surface, it is far more efficient to leave in lunar orbit what is needed for the return voyage to Earth, and then rendezvous with those resources only after having ascended from the landing base. In this diagram, the horizontal x-axis depicts distance while the vertical y-axis graphs the gravitational potential energy due to the Moon's mass.