PIN means Postal Index Number. PIN is used to identify the exact location of the Post Office to which address a letter is posted or is sent from. This helps the Postman to deliver the mail to the correct address.
There are nine PIN zones in India, including eight regional zones and one functional zone (for the Indian Army). The first digit of the PIN code indicates the region. The second digit indicates the sub-region, and the third digit indicates the sorting district within the region. The final three digits are assigned to individual post offices.
The PIN code system was started on 15th August 1972. The Department of Posts are very strict that all mails should clearly mention the PIN code so that the same can be delivered to the correct address with out delay.