The term "free content" refers to work which is available for reuse under a licence such as the Creative Commons Attribution-Share Alike License. People can not only copy free content, but also modify it and redistribute it (under the terms of the license). It is free as in freedom – the license grants freedoms that copyright normally denies.
A good reason to use Wikia is that all of the text on most of these sites is perpetually licensed under a free license. Thus, the content is free to use and will remain free.