A UI kit is a set of ready interface components (buttons, fields, cards, navigation) in a design tool – the visual library from which a designer assembles screens.
A UI kit is often confused with a design system, but it is only the system's visual part: components in a design tool without the code layer, pattern documentation or governance. A UI kit speeds up design; a design system connects design with implementation.
For smaller projects a well-built UI kit can be enough – provided it is used with discipline. Once more people or teams work on a product, the missing code layer starts to hurt: every developer interprets the mock-ups differently and consistency erodes.
In practice: start with a UI kit, and as the product grows, evolve it into a full design system with coded components.