The least-energy orbit is a term used when talking about interplanetary travel. When we wish to send an object into an orbit that goes from one planet to another we want the orbit we chose to use the least energy possible. Less energy means less fuel, which means less mass, which means a smaller lighter easier to build object, which means less money. The least energy orbit is the orbit where the aphelion of the orbit just touches one planets orbit and the perihelion just touches the others orbit. For example if we wanted to send a craft to mars using the least energy possible we would make the object orbit the sun so that on one side of the orbit (perihelion) it would just touch the earth's orbit, and on the other side (aphelion) it would just touch mars' orbit.