SIM lock - definition
SIM card can be locked using two different codes, one is known as PIN (Personal Identification Number) code and another one is known as PUK (Personal Unlocking Key) code. If a user sets any of these security codes and forgets afterwards then it is more likely unable to get back to working state until or unless user provides correct PIN/PUK codes. Every unique SIM card have unique PIN or PUK number. Things to be remembered that, after locking a SIM card using either any of them, if you try unlocking the SIM suing wrong PIN/PUK codes multiple times, could be more than 3/5/10 (depends on the carrier), then SIM card may get permanently blocked and under no circumstances it can be retrieved again without replacing the card. In that case you’ll lose all the stored information inside the SIM card.