Surface of zero velocity for the circular restricted three body problem. Basically trajectories of the system are required to stay on one side of this surface or the other.
The following image shows the contours of the surface. The two white areas represent the gravitational wells where the two masses lie. Three of the five fixed points for the system are shown: 2 fixed points are located in the dark oval shaped areas near the top and bottom of the figure, and one between the two masses in the middle of the X-shaped feature.