It doesn't mean free. It simply means that you can use a photo marked as royalty free multiple times in multiple projects. The price given is a basic price.

 

Most pictures displayed here can be bought in the two models. The difference is the target use and that target use defines a different price.

 

In whatever model, the user always needs to credit the author correctly and unambiguously so the material can be linked immediately to the author: "© Karel Overlaet - www.medianaut.be. All rights reserved."