When someone creates something (like a picture or book), they can make that work "open". This means that other people are allowed to copy it and change it if they want. Something that is open content may be free of charge, but it does not have to be.
The Simple English Wikipedia is open content. So are other Wikipedias. If a person changes open content or makes new open content, everyone can give it to anyone else, or even sell it. It is never needed to ask permission to do this, because the people who wrote the text already gave their permission when they clicked the "save" button.
The rules that say how people can use, change and pass around open content are called a license. A license explains exactly what you are allowed to do with the content that falls under it. Licenses are often written in difficult lawyer language (legal jargon), but many licenses have summaries that are much easier to understand.
The makers of open content get to choose what license to use for their work, and everyone else has to follow it. Only the maker, who owns the copyright, can change it to another license. Most open content licenses say that when others change the work, they must also declare it to be open and under the same license. This is called share-alike and means that anything based on work will always be open content.
All the content in Wikipedia is open under the rules of the Creative Commons Attribution/Share-Alike License, a very well-known open content license. Another well-known open content license is the GNU Free Documentation License.