This features allows you to manage all the variants of a product by differentiating them for color, size, image, ecc...
In Edit mode of the product you can add some variants. The variants can be for color, size and image. In that way, for example, you can specify different size for the same product.
This in an example of a product by differenting its for size in front-end.
You can find the variants in the filter in the search layout. This allows you to search a product by size.