A Lagrange point is a point in space around two orbiting bodies where another small body can remain at a relatively fixed orbit between the other two bodies. The forces of gravity due to those larger bodies at the point are equal, and they will provide the exact amount of centripetal force allowing the small body to rotate with them.
When any two large objects in an orbital configuration exist, there will be five Lagrange points, referred to as L1 through L5. These five points are all found in the orbital plane of the smaller body, as they are dependent primarily on the orbital path of such.
L1 exists between the two bodies; L2 lies at the point on the "dark side" of the smaller body, but along the same line as L1; L3 is on the opposite side of the smaller body's orbit, but, again, lies along the same line as L1 and L2; L4 and L5 form equilateral triangles between the larger body and the smaller, with L4 being prograde, or ahead of the smaller body in its orbit, and L5 being retrograde, or behind the smaller body in its orbit.