[llamabeast]

If someone attacks your province, the troops you will have in the battle will be any actual troops you have there, plus an extra force of a size determined by how much PD you have there. They will fight in the battle for you, but you can't see them apart from that. You can't move them around. If another enemy attacks, they will be there again, fully replenished. They are meant to represent local militias and defensive forces.

There is at least some explanation of PD in the manual. Also in the nation descriptions for each nation, it tells you what defenders you get for each point of PD (e.g. you might get an archer and a light infantry for every point). For PD values above 20 you start getting stronger, more elite troops present - but mostly PD above 20 is a waste of money anyway.