A Personal Identification Number (PIN) is mailed to each AdSense publisher when earnings have reached $50 on their account. This PIN helps Google AdSense to verify publisher addresses and protect the personal information of our publishers.
If a PIN has been mailed to you, Google AdSense will hold all payments until the PIN has been entered. However, your account will still be accessible, and you'll continue to be able to show ads and accrue earnings.
The PIN will be mailed to the payment address in your account.
REMEMBER: Google AdSense only give 180 days to enter your PIN within your publisher account, or to request a replacement. If you do not enter your PIN within this period, your account will be disabled and your earnings refunded to the appropriate advertisers.