SKU stands for "Stock Keeping Unit," it is a number or string of alpha and numeric characters that uniquely identify a product (in this case it is an intangible product such as a warranty).
SKUs are often called part numbers, product identifiers and product numbers. It may be a general number such as a UPC code or supplier part number or may be a unique identifier used by a specific a store or online retailer.
When you are shopping online or at retail stores, knowing the product's SKU can help you locate the exact product at a later time. It will help you detect a unique product when there are many similar options, such as a TV model that comes in different colors, sizes, etc.
Of interest, experts now suggest that many people don’t read it before shelling out for (usually) unnecessary extended warranties. If you read the manufacturer’s warranty first, you may discover that virtually everything offered by an extended warranty is already covered by the manufacturer. Learn more here.