when shows are "in public domain" what exactly does that mean? And why are some episodes of a series "in public domain" but other episodes of the same series aren't?

Public domain is when the intellectual property rights have expired or the owner failed to renew them. This is why you see the same 16 episodes of The Andy Griffith Show being packaged and repackaged into different looking sets each year along with the same 50 or so episodes of The Beverly Hillbillies.

Try doing a google search on copyright law.

I never knew what "public domain" meant until a few years ago researching the Three Stooges and always wondered why the same 4 shorts of their were always on bargain DVD's.

The same with the Fleisher Superman cartoons, they're in PD which is why you always see countless bargain releases on DVD in various cheap quality.