If you own a GSM type cellphone that you purchased at a discounted price when you signed up for service, the chances are that the cell phone company has 'locked' your phone so it will only work with their service.
If unlocked, you're free to use your choice of SIM and service with the phone, potentially saving you substantial money and adding to the flexibility of your phone.
you can use it with any GSM service provider and SIM, anywhere in the world.Phones are naturally unlocked, but a service provider might lock them
When originally manufactured, all GSM cell phones are unlocked. That is, they can be used with any SIM from any cellphone service provider - always assuming, of course, that the cellphone service provider has compatible GSM service on the same frequency bands that the phone can operate on. Most cell phone service providers
electronically 'lock' the phone so that it can only be used with their service. There are obvious reasons why they choose to do this - in particular, to force you to pay what are usually very high roaming charges when you take your phone out of their network and use it elsewhere in the world.
Fortunately, this locking is reversible. With some phones, it is possible to simply enter an unlocking password code into the phone and it is immediately then unlocked. Other phones need to be connected up to a special unlocking data terminal