it means you can put it on any GSM netowork across the world.
when you buy a GSM phone (one that takes a SIM card), it comes LOCKED to that carried, example if you buy a CINGULAR phone, you cant take that phone and use it on T-MOBILE's network, it wont accept a TMOBILE SIM.
my blackberry is unlocked meaning you can use it in europe, asia, canada, USA, south america, fuck use it on the moon if you can get a signal.
any carrier that uses GSM will work. and its a Quad band world device, so you can take it anywhere and it'll work