Early on, every screen is built from scratch. It works, until you have forty screens, three people, and buttons that look slightly different on each one. Things slow down and start to feel sloppy. A design system is how you stop that before it starts.
The simple version
A design system is a small box of ready-made, reusable parts: buttons, inputs, cards, colours, spacing. You build them once, properly, then snap them together to make any screen. Nobody reinvents a button, and everything matches because it came from the same box.
The analogy: Lego bricks
Lego does not sell you a finished castle. It sells you bricks that always fit together, so you can build a castle, then a spaceship, from the same set. A design system is your product's set of bricks. The parts are fixed and reliable; what you build with them is up to you.
How the box is organised
A good system has layers. At the bottom sit the raw decisions (colours, type, spacing). Those feed the components (buttons, inputs). Components combine into patterns (a form, a header). Patterns make the pages people actually use. Change something at the bottom and it updates everywhere above it, all at once.
Why it matters
A design system is what lets four people ship like fourteen. It removes the slow, repetitive decisions so the team spends its energy on the problems that are actually unique to your product. It pays for itself the first time you change one button and watch the whole app update with it.