Hi,

A pressure–volume diagram or PV diagram, or volume–pressure loop) is used to describe corresponding changes in volume and pressure in a system.

The product of pressure and volume is represented y an area on a P-V diagram. The area under the curve on a P-V diagram tells us the work done during the process. This can be found by adding up the areas of lots of thin rectangles, each of which corresponds to a constant (or very nearly constant) pressure.

Hope this helps.