The gist of what they are trying to say is that if you are in cover and declared hidden you cannot be intentionally targeted by any enemy for anything. You lose Hidden if someone can see *all* of you (not just part of you), or if someone gets within their Initiative distance. Other things cause you to lose Hidden, but they are in the rules and should be fairly easy to get.
You can use this if, e.g. your opponent is turtleing with ranged. You can use Hiding to leapfrog from cover to cover, hiding at the end of each move and even if he can see part of you he can't shoot you because you are Hidden. If you lose hidden because of anyone, everyone can now target you, not just the one that made you lose Hidden. If that makes sense.