Simplify the following text for a Grade 10 native English-speaking person.
Original: Content can be either in the public domain or under a license which allows re-distribution and re-use , such as Creative Commons Attribution and Attribution-Sharealike licenses or the GNU Free Documentation License ( GFDL ) . It is worth noting that the OKD covers open data as well as open content .
Simplified: All the content in Wikipedia is open under the rules of the GNU Free Documentation License , a very well-known open content license .