GTIN is a number used to uniquely identify trade items worldwide.
A trade item is any item of commerce (product or service) with specific characteristics that are essential for business transactions between partners anywhere in the supply chain.
Identifying and symbol-marking trade items automates the retail sales process through checkout (using price tag files), receipt of goods, inventory management, reorders, sales analysis, and a wide range of other commercial operations.
Examples: A can of paint sold to a consumer; a box of six paint cans; a container with 24 boxes of 1 kg of lawn fertilizer; a multi-pack with one shampoo and one hair conditioner.
For GTIN assignment rules, see here.