The term User Experience UX and User Interface UI is defined from the experience of a user in dealing with the app. The UI focuses on user friendliness from a graphical point of view (screens, process flow, etc.). The UI is part of the UX which is defined by the strategy of the app, the general design, the usability and many other factors.
The UX is crucial for the acceptance and success of a mobile application. Users decide whether they recommend it to others or not, based on their perception of the UX of an app.