Two bodies that orbit each other have five points of equilibrium between them. These are points at which the force of gravity is balanced with the centrifugal force between the two bodies. Whatever object is located at this equilibrium point is in a stable orbit. A ship located at a Lagrange point will need few course corrections to stay in orbit, and will save fuel compared to a ship in a normal orbit.
The Bobs frequently use the Lagrange points between a planet and its star in this way. By "parking" at the Lagrange point, the Bob is able to maintain his position relative to the planet without using much fuel.