It refers to ‘Universal Product Code’ assigned by the GS1 represented in a 12 digits barcode. It is a code assigned to the sellers that gives a unique identity to the company and its products. If you are a new seller, then obtaining a this is more important as without it, you cannot list the products. So, when browsing for the products, you can see a similar item sold by different sellers. Amazon distinguishes such products not by price or brand authenticity but with this unique code. The first step here is to register as a seller on amazon to fill the UPC and then get the product listed.