App UI refers to the user interface, or in other words, the way the app looks and feels. There are a few fundamental principles that designers should follow in order to achieve a good app UI. The aim is to create interfaces that are pleasurable and easy to use and this can be achieved by making the navigation intuitive and user friendly. Keeping the UI simple and avoiding too much clutter with unnecessary buttons or text can also improve your mobile app UI. Ideally, the designer should aim to only allow for 1-2 actions per screen. The placement and size of the buttons are equally important - the tap target should be within the thumb zone and large enough for each user to tap easily. All of the above, amongst other considerations, such as whitespace, typography, colours or language are important aspects of the mobile app UI and should be determined based on the target audience. Using user personas and customer journey maps informs you who you are designing the product for.