The Identity Protection PIN (IP PIN) prevents someone else filing a fraudulent return under your name and getting an undeserved refund.
If you had an IP PIN in the past, the IRS issues you a new one every calendar year, and you can't file your taxes electronically without it.
Theoretically they send the new one by snail-mail, but I have not received it this year and did not receive it in 2021 (2020 tax filing season) either.
However, you can retrieve the new one via the GET IP PIN tool, here:
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If you have not had an IP PIN in the past, but you want the added security, you can get one via the same tool.