“The Interplanetary Superhighway is a collection of gravitationally determined pathways through the solar system that require very little energy for an object to follow.
Each planet and moon has five locations in space called Lagrange points, where one body’s gravity balances another’s. Spacecraft can orbit there while burning very little fuel. The Interplanetary Superhighway maps out some possible flight paths among the Lagrange points, varying the distance the spacecraft would go and how fast or slow it would travel. Like threads twisted together to form a rope, the possible flight paths formed tubes in space.”

Read more about it here: www.jpl.nasa.gov/releases/2002/release_2002_147.html