User Interface is the visible and interactive elements a person uses to navigate and complete actions on a website or app.
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
In a booking form, the date picker, time choices, labels and confirmation button form the interface. A selected time should look different from an unavailable time, with a readable explanation when a choice cannot be made.