what is a restriction code on a nokia phone?

i broke my boost mobile phone so i took my sim card out to put in a nokia phone. when it asked my pin code i put it in. then it asked me a restriction code. what is it/

Many network providers lock phones to their network and sell them at low cost. When users buy such phones they need to stay with that network till the contract ends. In this way the network provider increases their potential. Incase a user needs to unlock their phone in the middle of contract they can request their respective network provider. If they deny there are many online unlocking vendors like Unlock-free.com who provide unlock (restriction code) for basic mobile models. You can use it and after unlocking the phone only GSM networks can be swapped.