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 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 network due to different regulation regarding comm. frequencies.