So, what's the license for these? http://www.nasa.gov/connect/artspace/participate/royalty_free_resources.html http://www.nasa.gov/connect/artspace/participate/

So, what's the license for these? http://www.nasa.gov/connect/artspace/participate/royalty_free_resources.html http://www.nasa.gov/connect/artspace/participate/royalty_fre... seems to indicate "royalty free". What does that mean in the context?