A Customizer is a way of grouping products that require similar inputs in order for you to produce them. You can add Customizers from your Maker Dashboard which allows you to add products to your store. Once the Customizer is published, you have the option to add new products under that Customizer. You can also think about Customizers as groupings of products from a Parent-Child relationship. The Customizer contains all of the options (Parent) whereas each product is a unique combination of those options (Child).