Basically, it means that the developer does not guarantee that the software works, because in some countries or jurisdictions (like the US) it is assumed that the developer provides a "warranty" that the software works unless the developer explicitly says otherwise.
So basically, it's like saying "you can't sue me if my software has bugs"
that make sense to me, but has this ever happened? has anyone ever downloaded some (in some way broken) software by an unpaid open source developer and then tried to sue that person?
it sounds mind-boggling to me that this would actually be possible, so I wonder if this was ever tried.