GTIN stands for Global Trade Item Number, also known as an EAN, barcode number or UPC-A. It is the 13-digit number below the stripy barcode on all consumer goods products. Smaller products may have an 8-digit GTIN, also known as UPC-C.
GTINs are the unique identifier for each one of your products that you will likely have linked to your own company's internal SKUs. QR Squared data is all managed up from the GTIN level, giving you unique insight into your products.
For further information on GTINs and how to order them for your business from GS1, please see the following GS1 web links.
Note that in the USA, the 12-digit GTIN is more commonly used.