In a broad sense, public domain images are images that the general public are permitted to use without restriction because no exclusive copyright restricts their usage (see also: Using Copyrighted Images). These images are usually quite old (expired copyright), created by the government, or released into the public domain by their creator. For a more comprehensive explanation, see Wikipedia's Public Domain article.
Star Army allows for the use of public domain images. Any unmodified Public Domain image must be placed in the by_license:public_domain: media namespace.
For example, this image is by NASA and is public domain:
Star Army is not a repository for images; public domain images should only be on Star Army if they're actually being used in articles.
These are freely-usable images. Many public domain images are in the public domain because they're old.
Note: Pixabay used to have public domain images but now they're under the Pixabay license so they should be treated as normal stock photos and placed in the by_license:pixabay namespace.