The visual elements (buttons, menus, screens) that let you interact with software. Like the dashboard and controls in a car that help you drive.
The home screen of your phone, with its icons and menus, is a user interface that helps you access different apps.
User Interface (UI) is a general software concept, not a single cloud service. All cloud providers offer many products with UIs (web consoles, dashboards, and app front ends), but there is no one-to-one “UI service” equivalent across AWS, Azure, Google Cloud, and OCI.