A Product represents a physical store keeping unit (SKU) that a customer can subscribe to. Products are put on Orders/Shipments to trigger logistics actions. Examples of Products are for example a certain bag of coffee, a bike, or any other (internal) product that needs to be communicated to a logistics or fulfilment process.
In a "smart order-based" subscription setup, the quantity and price of a Product is used to generate the line items on an Order.
In a "product as a service" or traditional recurring subscription setup, the quantity and price of a Product are used to calculate the amount that needs to be billed to the customer.
A generic or even per-customer replenishment/shipment schedule can be configured on a Product. For example, this allows you to configure Product to be shipped only once on signup or activation of a Subscription. Or it can be used to for example send razor blade replenishables to a certain customer every 6 weeks. The replenishable schedule for a Product can even differ per Subscription, allow you to offer personalized replenishment schema depending on your customer's needs.
Products can optionally be linked to a Plan so that multiple Products are immediately activated on a Subscription when someone signs up to a certain Plan.