GTIN stands for Global Trade Item Number, also known as an EAN, barcode number or UPC. It is the 13-digit number below the stripy barcode on all consumer goods products. It's the unique identifier for each one of your products, and Polytag data is all built at the GTIN level.
For further information on GTINs and how to order them for your business, please see the following GS1 web links
Note that GTIN-13's are the most commonly used GTIN, but there are also GTIN-8's (8 digits), GTIN-12's (12 digits) and GTIN-14's (14 digits).