Keeps others out. Or, if you set it up incorrectly, it could keep you in. So double-check before you sign off.
"Them." It means giant ants. They're the things you want to lock out. And if you're serious about "Them" being kept out, then you need a good lock.
Because, let's face it, locks aren't for "Us." "Us" can come and go as we want. We're all friends once we're "Us"! "Us" always get along! "Us" understand each other! And the moment "Us" doesn't understand each other? Then "Us" becomes "Them." And, as we know, "Them" get locked out.
Of course, it's also possible that we could simply rethink our personal classification systems and let go of our material needs and chance society to make "Us" and "giant ants" a thing of the past and instead live as "We" so as to avoid a constant and never-ending source of struggle between insiders and outsiders. But we can't make money doing that. So why bother? Be afraid of "Them" and protect "Us" and buy a good lock! You'll be happy you did.