The GFDL is a free documentation license that's designed, among other things, to be easy-to-use alongside the GPL and LGPL software licenses. Whenever you go to submit something on Wikipedia, Wiktionary, etc., there's a clause stating that you irrevocably agree to release your contribution under the GFDL.

The article 'Wikipedia:Copyrights' gets into more of what "most" means here if you genuinely want to reuse its material for your project. Notably, the amount of text that's actually been imported under CC BY-SA 4.0 and thus cannot be licensed under the GFDL is quite miniscule; I don't think I've ever seen this happen before.

(Notably this is not the case for Wikimedia Commons, where each piece of media has its own licensure.)

Why YSK: Probably a lot fewer people know about Wikimedia's usage of the GFDL than they do about Creative Commons (BY-SA 4.0). If you're making a project licensed under the GPL, LGPL, or GFDL, GFDL material will slot in much more elegantly than CC BY-SA 4.0, meaning you can easily reuse material from these major open knowledgebases.