UX is the User Experience. That is, what experience/impressions the user gets from working with your interface. Whether he can achieve the goal and how easy or difficult it is to do. And UI is the User Interface – what the interface looks like and what physical characteristics it acquires. It defines what color your "product" will be, will it be convenient for a person to put his finger on the buttons, will the text be readable, and so on... UX/UI design – is the design of any user interface in which usability is as important as appearance.