A SKU, or Stock Keeping Unit, is a unique number used to describe a product in an inventory system. SKUs are alphanumeric, and ideally provide information on the most important characteristics of a product. For phones, these characteristics are manufacturer, model, capacity, network, color, and variant.
SKUs in WholeCell are mainly helpful for two purposes:
Syncing your inventory with e-commerce channels and automatically committing inventory to e-commerce orders
Physically organizing your inventory in your warehouse by SKU
WholeCell, by design, can be used without creating SKUs for your products. But if you're trying to do either of the above, SKUs are the key.
In your SKU Settings page you can choose whether or not to enforce uniqueness among your SKUs. Enforcing uniqueness will prevent you from using the same SKU for multiple product variations.