You need your PUK to unblock the ID card PIN if you have entered it incorrectly three times. You will find the PUK for the German identity card, the residence permit and the eID card for EU citizens in your PIN letter. This letter will be sent to you by post approximately two weeks after you receive your ID card. In addition to your ID card PUK, the letter also contains your ID card PIN.
If you no longer have your PIN letter with your PUK at hand, you can also reset your ID card PIN without a PUK. You can find more information on resetting your ID card PIN here.