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.
If you want to save specialized information for your variants, then you can add custom fields to your variant details pages by using metafields. If you have an Online Store 2.0 theme, such as Dawn, the free Shopify theme, then you can add references to your product variant metafields by using the theme editor.