Preparing an experiment in a virtual laboratory you can start with the sample settings supplied with the program or set the parameters from scratches
Then you can run the simulations and observe the result in the windows. or - in some modules - also in full screen mode.
In the Gravity module, for example, you can investigate gravitational interactions of celestial bodies. You can define two, three or more objects like stars, planets, comets, space crafts etc.
The predefined samples will give you the whole Solar system at the state on Jan 1st, 2000, gravity catapults, the influence of Jupiter on the orbits of comets, the wobbling of stars caused by massive planets in tight orbits (one of the main methods of discovering extrasolar planets) etc.