From Wikipedia:
Free content (or free information) is any kind of functional work, artwork, or other creative content upon which no legal restriction has been placed that significantly interferes with people's freedom to use, understand, redistribute, improve, and share the content. Importantly, when free content is modified, expanded, or incorporated within another work, the resulting work must be legally distributable, either just as freely as or under more restrictive terms than the original. Free content, whether available at no cost or only for a fee, must be able to be changed if it may be used at all.
Practically speaking, free content encompasses all works in the public domain and also those copyrighted works whose licenses honor and uphold the freedoms mentioned above. Because the law by default grants copyright holders monopolistic control over their creations, copyrighted content must be explicitly declared free, usually by the referencing or inclusion of licensing statements from within the work.
A work in the public domain cannot be licensed because, by definition, its copyright has expired or been relinquished. However, such a work is still considered free content, because it may be used for any purpose whatsoever.