The simulation illustrates the situation of a person in an elevator. The elevator takes the person from one floor to the next floor up.
For this situation, try sketching three free-body diagrams, one for the person, another for the elevator, and a third for the person-elevator system.
First, draw the diagrams for when the system remains at rest. Then, predict whether the free-body diagrams will change (and, if so, how) when the elevator is accelerating up, moving up at constant velocity, and moving up but slowing down (acceleration is down).

The simulation draws the diagrams for all these cases, but make sure you try drawing your own before looking at the simulation's diagrams.