A dark UI is an interface where most of the background is dark and the text and controls are lighter. It is also called a dark theme or dark mode.
Some people prefer dark screens in certain settings, and some devices let an app follow the person's light-or-dark preference. A dark interface can feel calmer in a dim room, like reading a page with the lights turned low.
Dark does not automatically mean easier to read. Text and buttons still need enough contrast, and the design should work for people who choose a light theme too.