A Lagrange point is a point in space where the gravitational forces of two larger bodies, for example stars or planets, and the centrifugal force of a third, smaller object at that point cancel each other out. This allows the third object, for example a starship, to assume a fixed position relative to the other two objects without any power consumption.
Within star systems, several such points existed.
When a Delta Rana warship virtually appeared out of nothing in the proximity of Delta Rana IV, Riker assumed that the ship was riding a Lagrange point behind Rana IV's farthest moon. (TNG: "The Survivors")