When a customer visits a product page in your online store, the first available variant is selected by default, and the image for that variant is shown instead of the featured image. If you have a featured image that you would like to show instead of the first available variant image, then you can use this customization to disable the auto-selecting of the first available variant. That way, the customer will be prompted to manually select a variant before one is shown.
A featured image is a product image that belongs to the whole product, not just to a specific variant. For example, if you are selling a product that has variants in several different colors, then you might have a featured image that shows the different color options in one picture.