This feature works only if both Products and CRM plugins are installed.
In the deal's edit view there is a Deals items option with a plus icon. After you click on it, you will see a new blank line added. You can add here products for the deal. It is useful if you sell products or customer service.
If you click on New line - you will be able to fill in all the fields manually. If you type product name in Add product field - line will be filled in automatically. The sum cannot be changed manually, it is being calculated from lines.
Later, you can filter deals by Products and Product's category.