What does it mean when i iphone is unlocked?

I'm shopping for I phones online help

It can be used with any service provider SIM CARD.

For example when you get a 2 year contract with at&t and buy the phone as part of that contract, the phone is locked to at&t.

It basically means that you can put any sim card in and and it work. When a phone is locked it will only accept sim card from certain service provide like orange Vodafone or whoever it says it is locked to.

It means it can work on any GSM carrier in the world vs locked ones which can only work on one.

All the answers so far are correct.

It means that this phone can be used with any service provider- it is not locked to any specific network and is not on a contract. This phone can also be used internationally. So eg, you have an unlocked phone- it can be used with t-mobile, AT&T, Rogers, Orange, Digicel, O2, Vodaphone, etc etc and just about any carriers. And this is why unlocked phone is usually more expensive as it's restrictions has been lifted.

Hi, locked phone would only be able to connect to the original carrier's network with a SIM card from the same carrier. An unlocked iPhone can connect with any SIM card to any network around the globe.