0.0 is the EVE equivalent of the Empty Prairies -- except that it is far from empty.
Here CONCORD doesn't even try to protect people, and sovereignty is up for grabs.
It is also the home of the exotic ores and minerals, and of the miners who mine them, the (PC) pirates who prey on them (as well as the NPC rats, of higher levels, who prey on anyone they find), and the pirate-killers, in elite ships, who prey on pirates and rats alike. It is also where gate-camps and war fleets are commonplace, and where what few stations there are are, at the same time, valuable places of refuge and also obvious gank-points, since anyone wounded will be heading for them, and, once docked there, they must eventually, undock.
0.0 represents any security rating of 0.0 or below -- without checking specifically, there is no way of knowing how negative a system's security rating is.