Variants are set to differentiate one product from another on your online store.
You can set this from Inventory, Variants section of the 'Add Product tab'. Scroll down the page of the product to add and and configure the options for selling the product.
From here, You can set the price you will charge for your customers, Add the Stock Keeping Unit (SKU) code and the barcode as well.
If your product is available in different colours and sizes, you have the option to select the: this product has multiple options box at the bottom.