Open contentbook), you can choose to make that work "open", which means that other people are allowed to copy it and change it if they want.
If something is open content, it might be free, but it does not have to be. You can sometimes sell things that are open content.
The Simple English Wikipedia is open content. So are other Wikipedias. The content can be text or pictures or sound. If you change content or make new content, everyone can give or sell it to everyone else. They never need to ask you for your permission. When you click the save button, you give your permission right there.
However, you do insist that whenever else they sell or use your work, they also declare it to be open. The good turn gets passed on.