The GFDL is a Free Documentation License under the GNU project. If you use sites like Wikipedia for school or work, you are benefiting from the GFDL. Without a general open source license for this kind of content it may not be available to the public. When you contribute to Wikipedia pages you are generating GFDL content which others may use, Meant for text books and other documents, GFDL can be used for any document, as long as it conforms to a set of standards in use for nearly 15 years.
The GFDL (Gnu Free Documentation License) is not a good free content license because it is not suitable for all online content. Additionally, most people have a very difficult time understanding how to reuse content covered by the GFDL in any practical way. Even the FSF recognizes that there are safety concerns when using a GFDL.