Just in case, this is a developer's joke:
That file is actually a personnal DNS resolver, wich is responsible for pointing urls to computer ip's on the net, that's how you can type in
keyhero.com and end up here, as opposed to typing
https://45.33.89.92 (and still ending up here with a security warning on the certificate, but that's a lot less friendly notation).
So basically that file is read before your browser goes on to interrogate the world-wide DNS servers and says that
keyhero.com point to 0.0.0.0, wich doesn't exist so that will just get you a basic error page.
This is mainly used in web-development to point an address to your local computer (always 127.0.0.1), so you can work on your website without actually having to put it online.
It can also be used to get access to specific internal company tools that aren't published on the net and therefore are unknown to public DNS servers.
Of course you could also use it to pull a prank or two on your friends or colleagues, as in you could point
facebook.com to something else and tell them they've been hacked and watch as they loose their mind... but that would be mean ;p
