Correct me if i am not wrong they probably defining initial conditions for the particles (such as positions (to create the spongebob (althought I wish there was a way to convert an image to these n bodies), velocities, and masses) Then you set up the gravitational interactions between them and futher iteratively update their positions and velocities over time using some numerical integration method (Euler's method/Runge-Kutta method?) to simulate their motion