Most of the material I have put on this website is free to use. However the UK (& most of the rest of the rest of the world) defaults to material being copyrighted and fully restricted unless specified otherwise. Hence I had to specify that it is free in order to make it free.
There is another catch. I want to keep it free but if I made it fully public domain then there would be the risk that someone else could freely copy it, make some small modification, make the modified version the one people have to use and making that non-free. A solution is 'copyleft' which is the use of conventional copyright ownership to prevent republication or derivative works that are not similarly free to share.
I have used some readymade licences because it saves me work, they have been carefully legally worded and are familar to many people. I have used GPL & CC-by-sa. See the individual pages, programs etc. for which each is distributed under. I have used GPL a lot because it is long established and well known, even using it for documents where GFDL might be more optimised to the task, and CC-by-sa for artworks where GPL's requirement to make source code available might require people who use them to pass on full resolution copies of the image components they used and that would be (although desirable) inconvient given the size of such files and the difficulty of applying to print distributions.