universe sandbox calculates how objects are attracted by gravity or other forces. however, since it can't calculate at every instant of time, the positions and velocities of the objects, calculations are done once a timestep. the lower the timestep, the more frequently the calculations are done and the more accurate they are. the higher the timesteps, the less frequently and less accurate.
it is sometimes possible to predetermine objects paths using the projected paths tool, or if you want a basic orbit you can use the orbit controls to fine tune your orbit into the precise shape you want it.
also there is a habitable zone checkbox you can check which will create a green sphere shaped zone around a star, showing the habitable zone. this might not be exactly accurate though.