UI is user interface, the buttons, menus, forms, layouts, typography, colors, and interactive elements of a digital experience.
It matters because a website is often the first serious evaluation point for a buyer. Good design reduces confusion, builds trust quickly, supports accessibility, and helps visitors take the next step without feeling pushed.
Example
A UI designer specifies the appearance and behavior of a search field, its submit button and its results list. Focus, loading, error and empty-result states also need an intentional design.