Need an analogy?
An architectural concept of a home might only show primary rooms in the house (e.g. kitchen, living room, master suite). After the conceptual phase, the architect will define every single room in the house, from the laundry room to the coat closet.
Why does it matter?
Where design concepts typically only depict key hero screens in a product (e.g. dashboard, object page, etc.), production designs define every single page, interaction, and state, down to the click of a button.
How does it apply?
This is the phase where every minute detail is ironed out as the product is being built. It’s an important step in determining the product MVP, because certain aspects of a feature may or may not be in scope.