The simulation shows a beam resting on two triangular supports. There is a second copy below that, showing the free-body diagram of the beam. The beam in the free-body diagram will turn red if the situation is unstable, indicating that the beam is in danger of tipping over. (That can only happen when the 20-newton box is added, and positioned in a particular range of positions.) Note that you should see four sliders and two checkboxes. One checkbox turns on the center of mass, so a purple dot is shown at the location of the center of mass. The other checkbox places a 20-newton box on the beam, at a location determined by one of the sliders.
The counter has been running on this page since 8-10-2018. The number of people accessing the page since then is: