public Domainis commonly know as as intellectual property.That means it is "public property",that is available for anyone to use for any purpose. There is no control on anyone on it.The public domain often discussed in contrast to works whose use is restricted by copyright.Public domain books are only about 15% are in the public domain, even though only 10% of all books are still in print in market.
Public domain software is an app that will help you hunt down public domain works that you can use for site or blog content, info products, niche products, etc ...
Most of them are not share ware of any sort. You wll usually have to purchase the good ones that actually work.