This is not a real license, neither a copyright.

"Public domain" is a specialized term in copyright law that alludes to works not under copyright,
either in light of the fact that they were never in copyright in the first place (for instance, 
works created by U.S. government representatives, on government time and as an aspect of their 
responsibilities, are naturally in the public domain), or on the grounds that their copyright term
has at long last passed and they have "fallen into" the public domain.

To learn more:
https://opensource.org/faq#public-domain
https://en.wikipedia.org/wiki/Public_domain