What is an SKU?   

A Stock Keeping Unit (SKU) is an identification code used by a supplier to uniquely identify a billable item that could be either a product or a service. While they are used for physical products they can also be assigned to intangible items such as repair time or warranties.

To maintain the unique properties of the SKU a single code is often allocated to each and every variant of a product. One product can have many SKUs each identifying a particular variant right down to the last detail.

Using this highly specific code will ensure the product ordered is the one picked from the shelf when the order is fulfilled.

Sources: wikipedia