You add variants to a product that comes in more than one option, such as size or color. Each combination of options for a product is a variant for that product.
For example, suppose that you sell T-shirts with two options: size and color. The size option has three values: small, medium, and large. The color option has two values: blue and green. One specific variant from these options is a small, blue T-shirt.
You can see how many variants a product has in the Inventory column of the Products page.
For products that don't have any variants, the Pricing, Inventory, and Shipping sections are shown on the product details page. After you add variants, those sections are no longer shown on the product details page.