What does it mean when a cell phone is unlocked?

A cell phone is said to be unlocked if it can be freely used across multiple networks that are compatible with the phone's wireless technology. Having an unlocked phone enables users to easily switch between cellular networks.

A phone lock is essentially a software code the manufacturer places on the phone as per the request of the carrier selling the device. The lock guarantees that the phone can only be used within one carrier network by preventing it from accepting any other Subscriber Identity Module, or SIM, card. A phone can either be unlocked by simply entering an unlock code or by modifying the internal hardware of the phone.