When you open the interactive, you will see a cylinder filled with gas and topped with a piston. This is your thermodynamic "system". There is a large fixed graph that shows the pressure and volume of the gas. These values will change as you change conditions in the system via the slider bars and buttons in the controls section of the interactive below the cylinder. There is also a floating graph (so you can move it around) which tracks changes in quantities as a function of time, so that you can watch their evolution.