HTC HD7 unlocked GSM

What's an unlocked GSM on a phone? I have never heard about this before.
I am buying the new HTC HD7 before its release but I am confused because it says "unlocked GSM" and I don't know what that is or if its bad or good. Can I get some help on this one?

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It means you can use any SIM card on that phone. Locked phones are provider locked so you can only use SIMs from that provider...say AT&T or Sprint. If its unlocked then you can use any SIM around the world.