By clicking somewhere you place the Sun in that position and start a new simulation.
This applet shows the motion of the Earth (green) and Mars (blue) around the Sun (red).
In classical mechanics the three-body problem is to take the initial positions and velocities of three point masses
that orbit each other in space and calculate their subsequent trajectories using Newton's laws of motion and Newton's law of universal gravitation.
Three-body problem
Two-body problem