A collection of common components, principles and page patterns that help scale an individual product design, and eventually scale more digital products.
Here's an analogy.
Think of design systems like a glorified set of color swatches or design guidelines you would use to remodel your house. Rather than design every bedroom, you could define the color, door handles, and flooring for all bedrooms to save you time and ensure everything is consistent.
Why does it matter?
As digital products scale, the work involved in getting new screens and features into the product increases. Design systems help guide designers, developers and product managers so they can keep up without having to add more resources to the team.
How does it apply?
The concept of design systems has existed for decades in architecture and urban planning, but for digital products, the scale of growth is so fast that design systems are often the only way a product can scale while still being highly usable and consistent with brand standards.