You add variants to a product that comes in more than one option, such as size or color. Each combination of option values for a product can be 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. You can also manage inventory for each variant from the Inventory page.