Cindi Gerst you need to contact your provider. For example if your provider is T-Mobile you"
Contact Customer Care to request your PUK code.
When the device displays "SIM card is PUK-Locked. Please contact Customer Care," tap Emergency Call.
At the Enter PUK screen, enter the code provided by the Customer Care representative.
Tap OK.
Enter a new PIN.
Tap OK.
Confirm the new PIN.
Tap OK." this should be a similar process with all other providers.
As for yuor question about what it is "
A PUK or 'Personal Unlocking Key' is a security feature on most mobile devices that protects your SIM card data. A PUK code is required when a SIM Card PIN code has been entered incorrectly." from