Unlocked GSM phone means that it wouldn't be locked into a specific carrier network. On the upside, it means freedom to choose your own GSM carrier that suits your needs (and wallet) best. On the down side, several carrier-specific features may not be available.
If you lived in the US, make sure that the unlocked phone is compatible with US GSM frequencies. International phones or EU-spec phones may have problems connecting to US carrier networks due to different regulation regarding comm. frequencies.