A user interface is the series of screens, pages, and visual elements—like buttons and icons—that you use to interact with a device.
A UI designer is responsible for how product surfaces look and function. UI impacts the user experience, but only in one part of the customer journey. If a UI was the table, chairs, plates, and utensils in a restaurant then the UX would be everything from the food to the service, lighting, and music. Read more here.