The command assumes you downloaded and installed one of the beta utilities producted by the folks who are in the process of hacking the iPhone to unlock the subsidy lock that prevents the iPhone from being used with SIM cards from other carriers.
You should probably keep the discussion of the beta software tools over on the official forum, hacint0sh rather than here.
As with any beta software... this could cause bad things.
It sounds like most people who have bricked their iPhones have been able to do a recover, but it's vaguely possible you could screw it up in such a way that you can't do a recover from iTunes, and Apple can tell you broke it by hacking it and not replace it under warantee (say for instance, you bricked it when you changed all the graphics to pink, and you try to return it with a pink version of the "connect to iTunes to restore" graphic instead of the real one!
I'd insure it with homeowners before you hack it, just to be safe... and hope your homeowners will pay the claim without checking to see if you were hacking around on it.