How do u find out what a puk code is

Read this Apple Support article about how to use a SIM PIN --> Use a SIM PIN for your iPhone or iPad - Apple Support

Go to the section:

"If you forgot or don't know your SIM PIN

Never try to guess a SIM PIN or PUK code. The wrong guess can permanently lock your SIM card, and then you would need a new SIM card. If your device asks for a PIN [or PUK] code that you don't know, here's what to do:

1. Contact the carrier that gave you the SIM card. If you aren't sure which carrier to call, remove your iPhone SIM card [refer to: Remove or switch the SIM card in your iPhone - Apple Support ] or iPad SIM card [refer to: Remove or switch the SIM card in your iPad - Apple Support ] and check the card for the carrier's name or logo.

2. Ask your carrier to help you unlock your SIM card using the default SIM PIN or PUK code. 

3. If you can't unlock your SIM card using the SIM PIN or PUK code or if an alert says, "PUK exhausted," ask for a new SIM card."

*If you enter the wrong SIM PIN too many times, your wireless carrier might be able to give you a Personal Unlocking Key (PUK).

Read the support article for complete details.

A PUK is used to unlock a SIM which has had an incorrect SIM PIN entered too many times.

You can obtain a PUK from your carrier.

See:

Use a SIM PIN for your iPhone or iPad - Apple Support