Firstly how come a mobile phone gets 'locked'?What does it mean?
Every mobile phone is locked when it is purchased new, say that it was purchased with a Vodaphone sim card in it it is locked to Vodafone, you can get it unlocked (meaning that you could use any simcard eg. TMobile, O2, Orange etc. You can ge this done by the network itself in a shop or search for the unlock code on the internet.
Usually this refers to when a phone is bound or locked to a particular phone network. When you buy a phone, it will normally only work on one particular network. Phones can be unlocked to work on any network, either by contacting the network that a phone is locked to for an unlock code or by taking it to a phone repair/service shop or a local market place where they'll unlock it for you for a small fee. For some models of phone, the unlock codes can be found online.
Mobiles only work to one network such as O2, orange, vodafone, and will not work with any other sim card. By unlocking it you make it so that any sim card and therefore any network can be used. I'm not sure how it's done but i know it involves a computer. I had mine unlocked and took it to the market, they did it for ?10.
[it] restricts the use of these phones to specific countries and network providers
