Hi,
Right.
Let me explain you the differences.
In the order edition, you have two icon buttons, one with the "product" icon, and another with a "plus" icon.
The "product" icon allow you to browse your products and select an existing product to be added in the order.
The "plus" icon allow you to create manually a product which does not exist in your store (with the name you want, the price you want etc).
When you select an existing product, most field will be pre-filled in the popup you show in your screenshot, you will also have the product id displayed in the fist line (where you have "0" in your picture).
Hope it will help you.
Regards,