We defined work previously as:

W = F Dx     (true if the force is constant)

F = PA, so:

W = PA Dx = P D(Ax) = P DV

At constant pressure the work done by the system is the pressure multiplied by the change in volume.

If there is no change in volume, no work is done.

If there is a change in volume and the pressure changes the work done by the system is the area under the P-V graph. This is why P-V diagrams are so useful in thermodynamics.