The Berne Convention, of which most of the world is a signatory, states that copyright is assigned automatically to the creator upon the work's creation. Thus, if I create a piece of software, unless I specify otherwise, no one else is allowed to do anything with it.
Much of the time, this isn't what we want. Open-source licenses provide many freedoms to other people for our software.
However, most licenses are long and difficult to understand (which is why licensing is by far the most common topic here). The WTFPL exists to bypass the restrictions of the Berne Convention as simply as possible. That is all.
Does it also mean that the potential user can sue me for something happened because of the software I've written?
Per the faq:
By the way, with the WTFPL, can I also…
Oh but yes, of course you can.
But can I…
Yes you can.