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 a list of variants for a product on its product details page.
When you add variants to a product, it changes the way that you set their product details. For products that don't have any variants, you need to adjust the the Pricing, Inventory, and Shipping settings for the product from the product details page. After you add variants, you need to adjust those settings for each variant from the variant details page.
You can also manage inventory for each variant from the Inventory page.