By definition, a Stock Keeping Unit (or SKU) is a number assigned to a product by a retail store to identify the price, product options and manufacturer of the merchandise.
The SKU field is enabled by default in the catalog. You will find it on the product edit/add screen inside the product details box:
How to disable the SKU feature?
You can easily disable the catalog SKU feature in catalog general settings screen on the bottom. Please check the checkbox and save the settings to proceed: