A SIM lock, also known as a network lock or carrier lock, is a restriction placed on a mobile phone by the device’s carrier or service provider. When a phone is SIM-locked, it can only be used with a SIM card from the carrier that imposed the lock. The purpose of a SIM lock is to tie a mobile device to a specific network, typically as part of a contractual agreement or to ensure that the device is used exclusively on the carrier’s network.

The process of unlocking a phone involves removing the SIM lock so that the device can accept SIM cards from other carriers. Unlocking can be done through the carrier, often after the fulfilment of contract terms, or through third-party services. 

Unlocked phones provide users with the flexibility to switch carriers and use SIM cards from different providers, especially useful when traveling internationally or when seeking better service options.