UI design focuses on the user’s visual experience. It determines how a user interacts with an interface - be it an app, a video game or a website. It’s all about how the user navigates from A to B via different visual touch points. Think tapping a button or swiping through pictures.
The job of a UI designer is to design all the screens through which a user will move, and to create the visual elements - and their interactive properties - that facilitate this movement.
UI Designers think about what the user will expect and design the app’s interface accordingly.
In a nutshell: UX draws out the map, the bare bones. UI then fleshes them out with visual, interactive touch points that take the user through their journey as intuitively as possible.