We defined work previously as:

W = ∫ F dx

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.

We often sketch pressure-volume graphs when we're trying to understand a thermodynamic system. These graphs are helpful because the work done by the system is the area under the P-V graph.