UDIN refers to Unique Document Identification Number.
ICAI came up with a Unique Document Identification Number to prevent Non-CAs from impersonating CAs and providing certification. The system generates an 18 digit unique number for any document that a Chartered Accountant certifies.
In addition to this, the Regulators or Banks, or others can quickly check the genuineness of the certificates with the Unique Document Identification Number.