PUK stands for Pin Unlock Code. Your phone will only ask for this if you have locked your SIM by entering the incorrect PIN multiple times. When this happens, you’ll need to call your mobile network operator for the PUK code. Only they can give it to you.
You then have 10 chances to enter the correct PUK code. If you get it wrong 10 times, the SIM card will become permanently locked, and the only thing you can do now is request a new SIM from your network provider.