PUK stands for Personal Unlocking Key. It is unique to each SIM. If for whatever reason you have entered your PIN incorrectly 3 times, you will require a PUK to reset the PIN and allow you to use the phone.
Unfortunately Public Mobile does not provide the PUK on the SIM or the SIM packaging as other carriers have in the past. You therefore have to contact Public Mobile as noted by others. Here is a link to a PM help article on that, which also includes a link to get support: https://publicmobile.ca/en/bc/get-help/articles/where-to-find-your-puk
You should also be aware that as is shown on your son's phone, that you only have 10 attempts to properly enter the PUK. If you exceed that, the SIM will be disabled. You do not want that.