Public domain means, That you can use it!
When you use public domain materials, it means you are using something either old enough to be considered public or something that the author wanted to share with everyone.
Public domain does not apply to everything. Public domain only applies to those things that are over 100 years old. OR 50 to 70 years after the authors death. Also as stated above if the author wants everyone in the entire world to be able to use it the author can do that too.
Public domain allows people to be able to use works that have long since expired and use them to make works of art or build upon them to make them better. When people use PD, they generally use them in such a way as to educate people.