What is pleural parenchymal scarring?

What is Opacity at the left costophrenic angle most consistent with pleural parenchymal scarring?

[National Jewish Health]

It means that the opacity, commonly called a shadow, is probably just a scar, presumably from a past infection, perhaps even distant past.  The message is that this is almost certainly nothing to be concerned about.  The only action you might want to take is to have a repeat chest s-ray in about a year.