What's the meaning of ProductID that appears in invoice and estimates?

What's the meaning of ProductID that appears in invoice and estimates?

I was going through invoice api and i notice that 'ProductID" appears in both invoices and estimates. What exactly is a productid? I understand Item Id (because I'm able to create items in ZOHO invoice) but I'm wondering what is meant by ProductId. As part of Invoice user-interface, I don't find a way to create a product. Please clarify.