What is an unlocked phone?
A phone that isn't locked...
A phone that can work with any carrier Sprint Verizon At&t
An unlocked phone means that the phone has the ability of supporting more than one SIM card. You can use either the CDMA or GSM service providers. Read more from http://www.amazon.com/gp/feature.html?ie=UTF8&docId=566955, and http://www.ebay.com/gds/What-is-an-unlocked-cell-phone/10000000006138041/g.html.
pretty much what these guys said. Here is the Nexus 4. it comes already unlocked ready to work on any carrier. http://www.google.com/nexus/4/
Mean the phone can be use for more than one SIM card, both GSM or CDMA carrier service provider.